Methods of manufacture of advanced wafer bonded heterojunction bipolar transistors

ABSTRACT

Methods of manufacturing heterojunction bipolar transistors are described herein. An exemplary method can include providing an emitter/base stack comprising a substrate, a base over the substrate, and/or an emitter over the base. The exemplary method further can include forming a collector. The exemplary method also can include wafer bonding the base to the collector. Other embodiments are also disclosed herein.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Non-Provisional patentapplication Ser. No. 16/708,093, filed Dec. 9, 2019. U.S.Non-Provisional patent application Ser. No. 16/708,093 is a divisionalapplication of U.S. Non-Provisional patent application Ser. No.15/606,965, filed May 26, 2017. U.S. Non-Provisional patent applicationSer. No. 15/606,965 is a continuation of U.S. Non-Provisional patentapplication Ser. No. 14/504,114, filed Oct. 1, 2014.

U.S. Non-Provisional patent application Ser. No. 14/504,114 claims thebenefit of U.S. Provisional Patent Application No. 61/885,434, filedOct. 1, 2013. Further, U.S. Non-Provisional patent application Ser. No.14/504,114 is a continuation-in-part of U.S. Non-Provisional patentapplication Ser. No. 14/217,022, filed Mar. 17, 2014. U.S.Non-Provisional patent application Ser. No. 14/217,022 claims thebenefit of U.S. Provisional Patent Application No. 61/800,175, filedMar. 15, 2013, and of U.S. Provisional Patent Application No.61/885,434.

U.S. Non-Provisional patent application Ser. No. 16/708,093, U.S.Non-Provisional patent application Ser. No. 15/606,965, U.S.Non-Provisional patent application Ser. No. 14/504,114, U.S.Non-Provisional patent application Ser. No. 14/217,022, U.S. ProvisionalPatent Application No. 61/885,434, and U.S. Provisional PatentApplication No. 61/800,175 are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

This description relates generally to semiconductor devices, and moreparticularly to advanced heterojunction transistors and transistorlasers, and photonic devices.

BACKGROUND

Heterojunction transistors, including heterojunction bipolar transistors(HBTs), have been used for many decades. For example, HBTs using silicongermanium (SiGe) as the base layer for the HBTs and using silicon (Si)as the emitter layer for the HBTs have been used for a variety ofapplications, as have HBTs using gallium arsenide (GaAs) as the baselayer and aluminum gallium arsenide (AlGaAs) as the emitter layer.

Heterojunction devices are desirable for use as electronic and photonicdevices. Photonic devices can comprise: laser, transistor lasers,photodectors, and solar cells.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a general configuration of a bipolar transistor as a threeterminal device in its two constituent forms NPN and PNP.

FIG. 2 shows a general cross-sectional device depiction of a bipolartransistor in a vertical stack geometry.

FIG. 3 shows a flat band energy diagram for three typical heterojunctionsituations between the emitter and base materials: type I, type II, nearzero conduction band offset.

FIG. 4 shows a flat band energy diagram for three typical heterojunctionsituations between the base and collector materials: type I, type II,near zero conduction band offset.

FIG. 5 shows a flat band edge diagram through an NPN transistor foroptimized transport.

FIG. 6 shows a flat band edge diagram through a PNP transistor foroptimized transport.

FIG. 7 shows a general configuration of an injection laser diode, with aquantum well or quantum dot active region.

FIG. 8 shows a flat band energy diagram of a general configuration of aseparate confinement heterostructure laser with an active QW or QDregion which can be of the type I heterojunction band alignment.

FIG. 9 shows an example of a cut away device depiction of an edgeemitting injection diode laser.

FIG. 10 shows an example of a cross-sectional device depiction of aVCSEL, where light is coming out of the bottom, but could be designed sothat light comes out of the top.

FIG. 11 shows a simple diagram of an NPN transistor laser showing thethree terminal device configuration with a corresponding free carriertype designation, with a quantum well or quantum dot active region inthe base.

FIG. 12 shows a possible cross-sectional device depiction of an NPN edgeemitting transistor laser or light emitting structure.

FIG. 13 shows an example of a possible cross-sectional device depictionof an NPN VCSEL transistor laser where light is coming out of the top,but also could be designed so that light comes out of the bottom.

FIG. 14 shows energy band structure diagrams for Ge and Sn semiconductormaterials.

FIG. 15 shows a graph of collector current density J_(C) vs. turn-onvoltages (V_(BE)) of various HBT material systems.

FIG. 16 shows the hole concentration of GeSn films as a function of Sn%, as measured by Hall effect.

FIG. 17 shows a representative graph showing a GeSn direct bandgapenergy vs. its lattice constant.

FIG. 18 shows a range of wavelengths that can be achieved by direct gapGeSn vs. its lattice constant.

FIG. 19 shows a formation of quantum dot structures resulting fromself-assembled GeSn quantum dots by the Stranski-Krastanov (SK) methodthat transitions from two dimensional planar growth to island growth.

FIG. 20 shows a flat band energy diagram of a GeSn quantum dot to Siwhich can be of a type II heterojunction band alignment.

FIG. 21 shows a possible range of emission wavelengths that areachievable in a type II GeSn quantum dot heterostructure with Sibarriers. This possible data is for low Sn % GeSn alloys.

FIG. 22 shows a flat band energy diagram of GeSn (low Sn %) quantum dotwith SiGe barriers which can be of type I alignment.

FIG. 23 shows a methodology of planar growth of a QW region on top of abarrier layer, and then growth of another barrier layer on top of the QWlayer.

FIG. 24 shows a flat band energy band type I alignment of a GeSn QW witha GaAs barrier.

FIG. 25 shows a general flat band energy diagram of an NPN GeSn doubleheterojunction bipolar transistor.

FIG. 26 shows a flat band energy diagram that shows an NPN HBT with acompositionally graded GeSn base (Ge or low Sn % graded to higher Sn %GeSn).

FIG. 27 shows a resulting flat band energy diagram of a GeSn quantumwell or quantum dot material placed in the base region of aheterojunction transistor.

FIG. 28 shows a flat band energy diagram of a GeSn quantum well orquantum dot where a barrier has been graded from a base material to aGeSn active region.

FIG. 29 illustrates an exemplary flat band energy diagram of an NPNstructure of a GaAs emitter—GeSn base—GaAs collector symmetric doubleHBT.

FIG. 30 shows an exemplary cross-sectional device depiction embodimentof an NPN GaAs—GeSn—GaAs symmetric double HBT.

FIG. 31 illustrates an exemplary flat band energy diagram of an NPNstructure of a GaAs emitter—compositionally graded Ge—GeSn base—GaAscollector double heterojunction transistor where the base is graded GeSn(Ge or low Sn % graded to higher Sn % GeSn).

FIG. 32 shows an exemplary flat band energy diagram of an NPN transistorlaser structure with a GeSn QW or QD active region in a Ge P-typebase/barrier material 3200.

FIG. 33 shows a possible cross-sectional device depiction of an NPNtransistor laser structure with a GeSn QW or QD active region in a GeP-type base/barrier material 3300.

FIG. 34 shows an exemplary flat band energy diagram of an NPN transistorlaser structure with a GeSn QW or QD active region in a GaAs P-typebase/barrier material 3400.

FIG. 35 shows a possible cross-sectional device depiction of an NPN edgeemitting transistor laser structure with a GeSn QW or QD active regionin a GaAs P-type base/barrier material 3500.

FIG. 36 shows an exemplary flat band energy diagram of a SCH laserutilizing a GeSn QW or QD region located in UID Ge barrier/OCL regionwith GaAs cladding.

FIG. 37 shows a cross-sectional device depiction of a SCH laserutilizing a GeSn QW or QD region located in UID Ge barrier/OCL regionwith GaAs cladding.

FIG. 38 shows an exemplary flat band energy diagram of an SCH diodelaser utilizing a GeSn QW or QD region located in UID GaAs barrier/OCLregion with type InGaP cladding.

FIG. 39 shows a planar growth of strained GeSn (low Sn %) on Ge, with Gebarriers above and below a QW GeSn film.

FIG. 40 shows an island growth of strained GeSn (high Sn %) on Gebarrier layer with a subsequent formation of a QD layer.

FIG. 41 shows an exemplary flat band energy diagram of an NPN HBT withan ordered InGaP emitter, GeSn base, GaAs collector.

FIG. 42 shows an exemplary flat band energy diagram of an NPN HBT withan ordered InGaP N emitter, a graded GeSn (Ge or low Sn % graded tohigher Sn % GeSn) base, a GaAs N collector.

FIG. 43 shows an exemplary flat band energy diagram of an inverted NPNHBT structure where an emitter is grown first, a base material istensile strained Ge, a collector up structure.

FIG. 44 shows an exemplary flat band energy diagram of an inverted NPNHBT structure where an emitter is grown first, a base material istensile strained GeSn, a collector up structure.

FIG. 45 shows an exemplary flat band energy diagram of an NPNconfiguration where a compressively strained GeSn HBT collector grownfirst, emitter up.

FIG. 46 shows an exemplary flat band energy diagram of a GeSn Double HBTStructure graded Emitter and graded Collector grown first, where a GeSnbase can be compressively strained.

FIG. 47 shows an exemplary flat band energy diagram of an NPN AlGaAsemitter—GeSn base—GaAs collector double HBT.

FIG. 48 shows an exemplary flat band energy diagram of an NPN HBT laserwith AlGaAs emitter/cladding and AlGaAs collector/cladding.

FIG. 49 shows a possible exemplary flat band energy band diagram for asymmetric double heterojunction GeSiSn emitter—Ge base—GeSiSn collectorstructure which can work as an NPN or PNP transistor device.

FIG. 50 shows a possible exemplary flat band energy band diagram forGeSiSn emitter—graded GeSn (Ge or low Sn % graded to higher Sn % GeSn)base—GeSiSn collector structure double HBT 5000 which can work as an NPNor PNP transistor device.

FIG. 51 shows a possible exemplary flat band energy band diagram for asymmetric double HBT where a Ge QW or QD is embedded in a GeSiSn baseregion with SiGe emitter/cladding and SiGe collector/cladding.

FIG. 52 shows an exemplary flat band diagram of a Si emitter—SiGe basewith Ge QD or QW—Si collector light emitting HBT.

FIG. 53 shows a possible depiction of a cross-sectional device of a Sibased edge emitting transistor laser or light emitting structure.

FIG. 54 shows an additional variation of the laser structure of FIG. 52using Si_(0.6)Ge_(0.4) as a base/barrier material.

FIG. 55 shows an exemplary flat band energy diagram of a SCH laserutilizing a Ge QW or QD region located in UIDGe_(1-x)(Si_(0.8)Sn_(0.2))_(x) barrier/OCL.

FIG. 56 shows energy bandgaps of various semiconductors vs. theirlattice constant.

FIG. 57 shows an exemplary wafer bonding process that enables monolithicjoining of two dissimilar semiconductor materials.

FIG. 58 shows an exemplary flat band energy diagram of a wafer bondedNPN GaAs—GeSn—GaN HBT.

FIG. 59 shows an exemplary flat band energy diagram of an NPNGaAs—graded Ge—GeSn—GaN HBT.

FIG. 60 shows an exemplary cross-sectional device depiction of a waferbonded GaAs—GeSn—GaN/SiC NPN double HBT in a mesa configuration.

FIG. 61 shows a possible exemplary cross-section embodiment of a waferbonded GaAs—GeSn—GaN/Si NPN double HBT in a mesa configuration.

FIG. 62 shows QuantTera's wafer bonder configuration.

FIG. 63 shows a current-voltage characteristic of a wafer bonded P GeSnto N⁻ GaN showing PN rectifying behavior.

FIG. 64 shows an exemplary flat band energy band diagram of an NPN InGaPemitter—GeSn base—GaN collector double HBT.

FIG. 65 shows an exemplary flat band energy diagram of an NPNInGaP—graded Ge—GeSn—GaN HBT.

FIG. 66 shows a schematic methodology of the epitaxial lift off process.

FIG. 67 a shows a schematic of the top half of an HBT InGaP emitter/GeSnbase stack with the inclusion of an AlAs separation layer and the GaNcollector structure.

FIG. 68 shows a pre-processed top half of an HBT Top and the HF etch ofthe AlAS and ELO.

FIG. 69 shows where a top half of an HBT wafer is bonded to GaNcollector structure.

FIG. 70 shows an inverted top half of an HBT.

FIG. 71 shows the wafer bonding of an inverted top half of an HBT to aGaN collector structure.

FIG. 72 shows a cross-sectional device depiction of a fully wafer bondedHBT structure.

FIG. 73 shows an exemplary flat band energy diagram of an NPN GaAsemitter—Ge base—wurtzite GaN collector.

FIG. 74 shows an exemplary flat band energy diagram of an NPN GaAsemitter—Ge base—cubic GaN collector.

FIG. 75 shows various bandgap energies of semiconductor materials as afunction of lattice constant.

FIG. 76 shows a graph of collector current density J_(C) vs.base-emitter voltages (V_(BE)) of different heterojunction bipolartransistor technologies.

FIG. 77 shows an exemplary flat band energy diagram of an NPN GaAsemitter—Ge base—4H SiC collector.

FIG. 78 shows an exemplary flat band energy diagram of an NPN GaAsemitter—GeSn base—4H SiC collector.

FIG. 79 shows an exemplary flat band energy diagram of an NPN GaAsemitter—GeSn base—3C SiC collector.

FIG. 80 shows an exemplary flat band energy diagram of an NPN GaAsemitter—graded Ge to GeSn base—4H SiC collector.

FIG. 81 shows an exemplary flat band energy diagram of an NPN GaAsemitter—GeSn base—ZnSe collector.

FIG. 82 shows an exemplary schematic embodiment of an NPN GaAs—GeSn—ZnSedouble heterojunction bipolar transistor in a mesa configuration.

FIG. 83 shows an exemplary flat band energy diagram of an NPN GaAsemitter—graded Ge to GeSn base—ZnSe collector HBT.

FIG. 84 shows an exemplary flat band energy diagram of a separateconfinement heterostructure (SCH) laser utilizing a GeSn QW or QD regionlocated in UID Ge barrier/OCL region with SiGe layers.

FIG. 85 shows an exemplary flat band energy diagram of a transistorlaser structure Si emitter—SiGe base/barrier with GeSn QW or QD activeregion—Si collector.

FIG. 86 shows a possible exemplary schematic embodiment of a Si basededge emitting transistor laser or light emitting structure in a mesaconfiguration.

FIG. 87 shows a possible exemplary method of using pulsed laserdeposition (PLD) deposited GeSn and GeSiSn to promote adhesion andoptimize heterojunction for wafer bonding.

FIG. 88 shows a double crystal X-ray rocking curve for a GaNInAs layergrown on a GaAs substrate.

FIG. 89 shows room temperature photoluminescence (PL) measurements on aGaNInAs layer grown on a GaAs substrate.

FIG. 90 shows a spectral ellipsometric scan of a GaNInAs film grown onGaAs indicating that GaNInAs bandgap energy corroborates with PL data.

FIG. 91 shows a curve tracer result of current voltage characteristicfor P—GaNInAs/N—GaAs junction, which indicate that the turn-on voltageis low.

FIG. 92 shows a general configuration of a cross-sectional view of abipolar transistor utilizing a GaNInAs base in a vertical stackgeometry.

FIG. 93 shows a general configuration of an epitaxially grown multijunction solar cell and spectrum splitting to increase utilizing thefull spectrum of sunlight for solar energy.

FIG. 94 shows possible exemplary configurations of a multi junctionsolar cell structure based on InGaP, GaAs, GaNInAs, and Geheterojunction materials for high efficiency solar cells: (1) a triplejunction solar cell; and (2) a four junction solar cell.

For simplicity and clarity of illustration, the drawing figuresillustrate the general manner of construction, and descriptions anddetails of well-known features and techniques may be omitted to avoidunnecessarily obscuring the present disclosure. Additionally, elementsin the drawing figures are not necessarily drawn to scale. For example,the dimensions of some of the elements in the figures may be exaggeratedrelative to other elements to help improve understanding of embodimentsof the present disclosure. The same reference numerals in differentfigures denote the same elements.

The terms “first,” “second,” “third,” “fourth,” and the like in thedescription and in the claims, if any, are used for distinguishingbetween similar elements and not necessarily for describing a particularhierarchical, sequential, or chronological order. It is to be understoodthat the terms so used are interchangeable under appropriatecircumstances such that the embodiments described herein are, forexample, capable of operation in sequences other than those illustratedor otherwise described herein. Furthermore, the terms “include,” and“have,” and any variations thereof, are intended to cover anon-exclusive inclusion, such that a process, method, system, article,device, or apparatus that comprises a list of elements is notnecessarily limited to those elements, but may include other elementsnot expressly listed or inherent to such process, method, system,article, device, or apparatus.

The terms “left,” “right,” “front,” “back,” “top,” “bottom,” “over,”“under,” and the like in the description and in the claims, if any, areused for descriptive purposes and not necessarily for describingpermanent relative positions. It is to be understood that the terms soused are interchangeable under appropriate circumstances such that theembodiments described herein are, for example, capable of operation inother orientations than those illustrated or otherwise described herein.

The terms “couple,” “coupled,” “couples,” “coupling,” and the likeshould be broadly understood and refer to connecting two or moreelements or signals, electrically, mechanically or otherwise. Two ormore electrical elements may be electrically coupled, but notmechanically or otherwise coupled; two or more mechanical elements maybe mechanically coupled, but not electrically or otherwise coupled; twoor more electrical elements may be mechanically coupled, but notelectrically or otherwise coupled. Coupling (whether mechanical,electrical, or otherwise) may be for any length of time, e.g., permanentor semi-permanent or only for an instant.

Electrical coupling” and the like should be broadly understood andinclude coupling involving any electrical signal, whether a powersignal, a data signal, and/or other types or combinations of electricalsignals. “Mechanical coupling” and the like should be broadly understoodand include mechanical coupling of all types. The absence of the word“removably,” “removable,” and the like near the word “coupled,” and thelike does not mean that the coupling, etc. in question is or is notremovable.

DETAILED DESCRIPTION

The fabrication of a germanium tin (GeSn) or germanium (Ge) or galliumnitride indium arsenide (GaNInAs) based heterojunction bipolartransistor and/or light emitting transistor or transistor laser or lightemitting device or laser or light absorbing for electronics andphotonics is described herein. Where GeSn is used as the base materialin a heterojunction transistor, the GeSn, GeSn quantum dot, GeSn quantumwire, and/or GeSn quantum dot material can be used as the active regionof a light emitting transistor or transistor laser or light emittingdevice or laser. Also where Ge is used as the base material in aheterojunction transistor, the Ge, Ge quantum dot, Ge quantum wire,and/or Ge quantum dot material can be used as the active region of alight emitting transistor or transistor laser or light emitting deviceor laser. In one embodiment, a heterojunction bipolar transistorincludes a Ge base region. In another embodiment, a method ofmanufacturing a heterojunction bipolar transistor includes forming aGeSn base region. Additionally in another embodiment, a method ofmanufacturing a heterojunction bipolar transistor includes forming a Gebase region. One additional embodiment, a method of manufacturing aheterojunction device includes forming a GaNInAs low bandgap materialfor use in transistors, wireless devices, lasers, photo-detectors, andsolar cells. Note that Ge can be interchanged with GeSn for a variationof the embodiments of the devices elucidated. In a further embodiment, adevice includes: a first heterojunction bipolar transistor comprising aPNP device having a first GeSn base; and a second heterojunction bipolartransistor comprising an NPN device having a second GeSn base, whereinthe first and second heterojunction bipolar transistors are located overa common substrate. In another embodiment, method of manufacturing adevice includes: forming a first heterojunction bipolar transistorcomprising a PNP device having a first GeSn base; and forming a secondheterojunction bipolar transistor comprising an NPN device having asecond GeSn base, wherein forming the first and second heterojunctionbipolar transistors occur simultaneously with each other over a commonsubstrate. In yet another embodiment, a device includes: a first bipolartransistor comprising a first GeSn base; and a second bipolar transistorcomprising a second GeSn base, wherein the first and second bipolartransistors are complementary devices and are located over a commonsubstrate. In a further embodiment, a method of manufacturing a deviceincludes: forming a first bipolar transistor comprising a first GeSnbase; and forming a second bipolar transistor comprising a second GeSnbase, wherein forming the first and second bipolar transistors occursimultaneously with each other over a common substrate. In still anotherembodiment, a bipolar transistor includes a GeSn base region, and in yetanother embodiment, a method of manufacturing a bipolar transistorincludes providing a GeSn base region. In a further embodiment, atransistor laser includes a GeSn active region, and in anotherembodiment, a method of forming a transistor laser includes forming aGeSn active region. The description herein elucidates a methodology formaking a heterojunction bipolar transistor (HBT) that utilizes GeSn asthe base material. Furthermore, the unique properties of GeSn can beutilized as the active region of a variation of the transistor which isthe transistor laser, or in a light emitting device like a laser.Embodiments described herein can relate to the following: GeSn which hasthe smallest band gap energy for the material systems GaN, GaAs, Si,InP, Ge, Sn, AlAs, InAs, GaP, SiC, ZnSe, etc., and thus would be usefulfor making a heterojunction bipolar transistor, laser or transistorlaser device. Additionally embodiments described herein can relate to Geor strained Ge and GaNInAs which have small bandgap energies.Nomenclature: Ga (gallium), N (nitrogen or nitride), As (arsenic orarsenide), Si (silicon), In (indium), P (phosphorous or phosphide), Ge(germanium), Al (aluminum), Sn (tin), Sb (antimony or antimonide), B(boron), C (carbon, carbide), Zn (zinc), and Se (selenium, selenide).The embodiments can relate to the following:

-   -   1) Bipolar transistor using a Ge or strained GeSn base.    -   2) Bipolar transistor using a graded Ge—GeSn base.    -   3) Bipolar light emitting transistor or transistor laser using        in the base region a strained Ge quantum well, quantum dot, or        GeSn quantum well, quantum wire, or quantum dot active region.    -   4) Light emitting or laser structure using GeSn or strained Ge        quantum well, quantum wire, or quantum dot active region.

The same or different embodiments can relate to:

-   -   1) Lateral structures (edge emitting).    -   2) Vertical structures.    -   3) Inverted vertical structures.    -   4) Indirect band gap GeSn.    -   5) Direct band gap GeSn.    -   6) Indirect gap Ge.    -   7) Strained Ge: tensile or compressively strained.    -   8) Single or Multiple quantum well layers.    -   9) Single or Multiple quantum dot layers.    -   10) Single or Multiple quantum wire layers.

In one embodiment, a heterojunction bipolar transistor can include aGeSn base region. In another embodiment, a heterojunction bipolartransistor can include a Ge base region. In another embodiment, a methodof manufacturing a heterojunction bipolar transistor can include forminga GeSn base region. In a further embodiment, a device can include afirst heterojunction bipolar transistor comprising a PNP device having afirst GeSn base, and a second heterojunction bipolar transistorincluding an NPN device having a second GeSn base.

A bipolar transistor or bipolar junction transistor is a three terminalor three layer device that relies on doping (adding “impurity” atoms) ofthe semiconductor layers to form N-type or “N” (electron surplus layer)semiconductor and P-type or “P” (electron deficient layer) semiconductorto form PN junction (diodes) in a three terminal configuration. Thisthree terminal or three layer device can include back-to-back PNjunctions to form a three layer sandwich with each of the layers namedthe emitter, base, and collector. There are two kinds of bipolartransistors, NPN and PNP.

For simplicity and clarity of illustration, the drawing figuresillustrate the general manner of construction, and descriptions anddetails of well-known features and techniques may be omitted to avoidunnecessarily obscuring the present disclosure. Additionally, elementsin the drawing figures are not necessarily drawn to scale. For example,the dimensions of some of the elements in the figures may be exaggeratedrelative to other elements to help improve understanding of embodimentsof the present disclosure. The same reference numerals in differentfigures denote the same elements.

FIG. 1 shows a general configuration of a bipolar transistor as a threeterminal device in its two constituent forms NPN 0101 and PNP 0108. TheNPN 0101 structure comprises an N-type emitter 0102, connected to aP-type base 0103, which is then connected to a N-type collector 0104region, which comprises the three terminal device. The correspondingcurrents in the three terminal device correspond to the emitter currentI_(E) 0105, base current I_(B) 0106, and collector current I_(C) 0107.The NPN 0101 device has a junction at the emitter-base, where theapplied voltage is V_(BE) 0115, and a second junction at thebase-collector, where the applied voltage is V_(BC) 0116.

The PNP 0108 structure comprises a P-type emitter 0109, connected to anN-type base 0110, which is then connected to a P-type collector 0111region, which comprises the three terminal device. The correspondingcurrents in the three terminal device correspond to the emitter currentI_(E) 0112, base current I_(B) 0113, and collector current I_(C) 0114.The PNP 0108 device has a junction at the emitter-base, where theapplied voltage is V_(BE) 0117, and a second junction is at thebase-collector, where the applied voltage is V_(BC) 0118. Thebase-emitter voltage V_(BE) turns on the transistor and generally isoperated in forward bias, and the base-collector voltage V_(BC) isgenerally reversed biased and also determines the breakdown voltage ofthe device.

The base region controls the operation of the transistor. Thecharacteristics and properties of the base material and the base-emitterjunction and the base-collector junction are the dominant factors thatdetermine the electronic properties of bipolar and heterojunctionbipolar transistors. Thus the utilization of a new type of base materialin these transistors allows for the development of vastly improvedtransistors for high speed and power efficient operation.

Semiconductors can be discussed in terms of their energy band structure.The energy band structure shows the allowable carrier (electron or hole)energy states for semiconductor as a function of the crystal momentumdirection. The energy band structure can be divided into two mainregions: the conduction band; and the valence band. N-type materialconduction relies on free movement of electrons in the conduction bandof the material. The conduction band can be characterized by theconduction band energy level (lowest energy in the conduction band).P-type material conduction relies on the free movement of holes (hole:absence of an electron) in the valence band of the material. The valenceband is characterized by the valence band energy level (or the highestenergy level in the valence band). The difference between the conductionband energy level and the valence band energy level determine the energyband gap of the semiconductor (difference of the conduction band energyminima to the valence band energy maxima).

At the PN junctions of the bipolar transistor, there exists a depletionzone that in the absence of an externally applied electric fieldprevents the movement of the charge carriers across the junctions ordifferent layers. The operation of this device relies on two types ofcarriers, free electrons (negative charges in the conduction band) andfree holes (absent electron charge carrier, positive charge in thevalence band). Thus, the name bipolar is ascribed to this device becauseits operation involves both electrons and holes, as opposed to unipolardevices like field effect transistors whose operation involves only oneof electrons or holes.

The bipolar transistor has three distinct regions: the emitter, thebase, and the collector. The flow of charges (called electrical currentor current) in this transistor is due to the bidirectional diffusion ofcharge carriers across the junction. The bipolar transistor is biased asfollows. The emitter is forward biased via the contact pads with thevoltage potential (base-emitter voltage V_(BE)) to force charge carriersfrom the emitter to the base. The collector is reversed biased via thecontact pads with a voltage potential (base-collector voltage) thatcauses charge carriers to be attracted from the base to the collector.The corresponding currents are called the emitter current, base current,and the collector current.

FIG. 2 shows a general configuration of a bipolar transistor as avertical stack geometry. Typically the structure can be grownepitaxially, ion implanted or fabricated by various means. For avertical bipolar transistor, typically a conducting or insulatingsubstrate 0201 is used as the seed crystal to start the growth of thestructure. A highly conducting sub-collector 0202 is then grown,followed by a low doped collector 0203. The base 0204 which is ofopposite conductivity as the collector 0203 is then grown, followed byan emitter 0205, and finally a highly conducting contact layer 0206.Electrical contact is made to device via the metalized contact pads:emitter contact 0207, base contact 0208, and collector contact 0209. Thevoltages and currents are applied to the device via the contact pads.Vertical configuration offers some advantages.

Some of the advantages of a bipolar device are: typically in an NPNconfiguration electrons will travel vertically in the device from theemitter to the collector. Thus it is straightforward to produce deviceswhere the electron transit time through the device is short (high cutoff frequency F_(t)). Generally the entire area of the emitter contactwill conduct the current; thus one can have high current densities in asmall area, thus allowing for high circuit densities. The turn-onvoltage (voltage across the base-emitter junction) V_(BE) is independentof device processing issues like size because it corresponds to thepotential at the base-emitter junction, thus process variations across awafer can be minimized which is desirable for manufacturing. The turn-onvoltage V_(BE) controls the output current at the collector I_(C) andresults in a high transconductance g_(m)=eI_(C)/k_(B)T, where “e” is thecharge of the electron, “I_(C)” is the collector current, “k_(B)” isBoltzmann's constant, and “T” is the temperature. This is the highesttransconductance available for any three terminal device and allowscircuit operation with low V_(BE) swings.

The operation of the bipolar transistor (transistor action) is based onthe flow of charge carriers injected from the emitter into the basewhich can diffuse into the collector forming the emitter to collectorcurrent (collector current). The free charge carriers initially in theemitter are called majority carriers. The majority charge carriers thatare injected into the base from the emitter, once in the base, arecalled minority carriers, which then can diffuse to the collector. Thebase region controls the flow of the minority carriers injected into thebase thus controlling the flow of the collector current from the emitterto the collector. By drawing out the minority carriers that are injectedinto the base from the emitter, small levels of minority carriers drawnfrom the base control the larger collector current that flows from theemitter to the collector. Also, the base region is made thin to enhancethe diffusion of carriers from the emitter to the collector.

The current gain or “beta” of the bipolar transistor is the ratio of thecollector current I_(C) to the base current I_(B). Basically, the ratiois the number of carriers that get across the transistor from theemitter to the collector, vs. the number of carriers that get caught inthe base.

For a typical NPN transistor, the biasing scheme is as such. The emitterto base V_(BE) bias is such that the base is biased slightly positive ascompared to the emitter. The collector to base V_(BC) bias is such thatthe collector is biased much more positively than the base. This biasingscheme can ensure that, for small values of base current, large valuesof collector currents can be controlled. The current gain is typicallyabout “100”.

For a typical PNP transistor, the biasing scheme is as such. The emitterto base V_(BE) bias is such that the base is biased slightly negative ascompared to the emitter. The collector to base V_(BC) bias is such thatthe collector is biased much more negatively than the base. This biasingscheme can ensure that, for small values of base current, large valuesof collector currents can be controlled.

The bipolar homojunction transistor can be made of one semiconductormaterial. The bipolar transistor can be used as a switch, amplifier, oroscillator, etc. It can be fabricated in discrete (single) component oras a component in integrated circuits.

Heterojunction bipolar junction transistors (HBT) can differ from thebipolar transistor (also called a homojunction bipolar transistor) byusing at least two different semiconductors. The heterojunction bipolartransistor typically uses different semiconductor materials for at leastone of the junctions, the emitter-base junction, and/or thebase-collector junction. The use of differing semiconductor materials iscalled a heterojunction.

Heterojunction bipolar transistors (HBTs) can be advantageous in somesituations for formation of the emitter-base junction. In homojunctiontransistors, the emitter is typically doped (impurity incorporation withan atomic element to create free charge carriers) more heavily than thebase region. If the heterojunction is designed properly, the emitter hasan energy bandgap greater than the base region. If the conduction andvalence band alignment of the two materials that form the heterojunctionis proper, it is then possible to limit the injection of majoritycarriers (initial free charge carriers in the base) of the base regioninto the emitter region (or additionally termed, limit the minoritycarrier injection into the emitter). This occurs because theheterojunction can create a potential barrier either in the valence orconduction band to block the majority carriers in the base, thuseliminating injection of majority carriers from the base into theemitter. In the heterojunction transistor, the base can be heavily dopedat concentrations much greater than the emitter material. The physics ofthe heterojunction can be strongly determined by the conduction andvalence band alignment between the materials.

FIG. 3 shows a flat band energy diagram for three typical heterojunctionsituations between emitter and base materials (focuses on the case wherewe have an N-type emitter material and a P-type base material): type I,type II, and near zero conduction band offset. The heterojunction is atthe interface between the emitter material and the base material. Thetype of diagram depicted is called a flat band energy diagram, whichrepresents how the conduction band edge 0310 and the valence band edge0311 change as one goes through the dissimilar semiconductor materials.The vertical axis 0312 has the value of Energy with typical units of“eV”, and the horizontal axis 0313 is the relative Distance in arbitraryunits “A.U.” through the heterojunction of semiconductor materials. Theenergy level line called the conduction band edge 0310 is the minimumenergy value of the conduction band, and the energy level line calledthe valence band edge 0311 is the maximum value of the valence band. Thediagram shows distance on the horizontal scale and that is a relativedistance into the semiconductor device. One could put units ofthickness, but that is usually omitted, and this represents a schematicfor carrier transport. The difference between the conduction band edge0310 and the valence band edge 0311 in the base material is the bandgapenergy of the base material. The difference between the conduction bandedge 0310 and the valence band edge 0311 in the emitter material is thebandgap energy of the emitter material.

There are various types of heterojunctions between the emitter and basematerials: (1) Type I heterojunction 0301; (2) Type II heterojunction0302; and (3) near zero conduction band offset 0303 (there can also be anear zero valence band offset) at the heterojunction. Type Iheterojunction has an energy discontinuity at the conduction band andvalence band, where the smaller band gap at base material 0305 liesbetween the conduction and valence band edges of emitter material 0304.ΔE_(C) is called the conduction band offset at the emitter-baseheterojunction (difference between the conduction band edges in therespective materials), and ΔE_(V) is called the valence band offset atthe emitter-base junction (difference between the valence band edges inthe respective materials). Type II heterojunctions have a discontinuityat the conduction and valence band edge, but the base energy alignmentis staggered or offset. The energy bandgap of base material 0307 can bestaggered above emitter material 0306, and the bandgap as depicted inthe figure (or staggered below the emitter material 0306 bandgap).Finally, one can have a situation of a zero or near zero conduction bandoffset heterojunction 0303 as shown in the figure, where the conductionband offset ΔE_(C) is zero or small, typically less than 0.1 eV betweenemitter material 0308 and base material 0309.

For NPN heterojunction transistors, where the emitter material is N-typeand the base material is P-type, a large valence band offset ΔE_(V)between the emitter and the base is desired, as shown in the threecases: (1) Type I heterojunction 0301; (2) Type II heterojunction 0302;and (3) near zero conduction band offset heterojunction 0303. This largevalence band offset ΔE_(V) prevents the back injection of holes from thebase to emitter, which can reduce the gain of the transistor. Thus thebase material bandgap energy should be less than the emitter materialbandgap energy. Looking at the FIG. 3 diagram, it seems that a type IIheterojunction current 0302 would be preferable because it has thelargest valence band offset ΔE_(V), but in some examples, the desiredsituation for the efficient transport of carriers across the base topromote transistor action is the near zero conduction band offsetheterojunction 0303 situation or where the conduction band offset ΔE_(C)is typically less than 0.1 eV.

FIG. 4 shows a flat band energy diagram for three typical heterojunctionsituations between the base material and the collector material (focuseson the case where we have a P-type base material and an N-type collectormaterial): type I, type II, and near zero conduction band offset. Theheterojunction is at the interface between the base material andcollector material. The type of diagram depicted is called a flat bandedge energy diagram, where the vertical axis is the Energy (eV) 0412value and the horizontal axis is a relative Distance (A.U.) 0413 throughheterojunction of semiconductor materials. The energy level lines arecalled the conduction band edge 0410, is the minimum energy value of theconduction band and the valence band edge 0411 is the maximum value ofthe valence band. The diagram shows distance on the horizontal scale,and the distance is a relative distance into the semiconductor device.One could put units of thickness, but that is usually omitted and thisrepresents a schematic for carrier transport. The difference between theconduction band edge 0410 and the valence band edge 0411 in the basematerial is the bandgap energy of the base material. The differencebetween the conduction band edge 0410 and the valence band edge 0411 inthe collector material is the bandgap energy of the collector material.

There are various types of heterojunctions between the base andcollector materials: (1) Type I heterojunction 0401; (2) Type IIheterojunction 0402; and (3) near zero conduction band offset 0403(there can also be a near zero valence band offset) at theheterojunction. Type I heterojunction has an energy discontinuity at theconduction band and valence band, where the smaller bandgap basematerial 0404 regions lies between the conduction and valence band edgesof the collector material 0405. ΔE_(C) is called the conduction bandoffset at the base-collector heterojunction (difference between theconduction band edges in the respective materials), and ΔE_(V) is calledthe valence band offset at the base-collector heterojunction (differencebetween the valence band edges in the respective materials). Type IIheterojunctions have a discontinuity at the conduction and valence bandedge, but the base energy alignment is staggered or offset. The energybandgap of base material 0406 can be staggered above the bandgap ofcollector material 0407 as depicted in the figure (or staggered belowthe bandgap of collector material 0407). Finally, one can have asituation of a zero or near zero conduction band offset heterojunction0403 as shown in the figure, where the conduction band offset ΔE_(C) iszero or small, typically less than 0.1 eV between the base material 0408and the collector material 0409.

For NPN heterojunction transistors, where the base material is P-typeand the collector material is typically N-type, one would like a largecollector bandgap energy because this allows the NPN transistor to havea big breakdown voltage. Looking at the FIG. 4 diagram, it seems that atype II heterojunction 0402 would be preferable because the basematerial 0406 has higher conduction band energy than the collectormaterial 0407, but in some examples, the desired situation for theefficient transport of carriers across the base to promote transistoraction is the near zero conduction band offset heterojunction 0403situation or where the conduction band offset ΔE_(C) is typically lessthan 0.1 eV.

FIG. 5 shows a flat band energy diagram through an NPN heterojunctionbipolar transistor 0500 for optimized transport. The figure shows thelineup of the conduction band edge 0504 and the valence band edge 0505through the NPN heterojunction transistor 0500. There is a zeroconduction or near zero conduction band edge offset ΔE_(C) from the Nemitter material 0501 to the P base material 0502 to the N collector0503. The emitter-base valence band offset is represented by ΔE_(VE) andthe base-collector valence band offset is ΔE_(VC). In physicalsituations it is desirable to have the smallest conduction band offsetthat is possible from N emitter material 0501 to P-base material 0502 toN collector 0503. Here the bandgap energy of N emitter material 0501 islarger than the bandgap energy of P base material 0502, where there is alarge valence band offset ΔE_(VE) between N emitter material 0501 to Pbase material 0502, and the junction is a heterojunction. The bandgapenergy of N collector material 0503 can be less than, equal to, orgreater than the bandgap energy of P base material 0502. Generally thebandgap energy of N collector material 0503 should be equal to, orgreater than the bandgap energy of P base material 0502. The greater thebandgap energy of N collector material 0503 the better the breakdownvoltage of NPN heterojunction bipolar transistor 0500. This is generallydesirable for high power and robust devices. If N collector material0503 is the same material as P base material 0502, there is ahomojunction at the base-collector junction and a heterojunction at theemitter-base junction, and such a device is called a singleheterojunction bipolar transistor device. If N emitter material 0501 andN collector material 0503 are the same, then the device is called asymmetric double heterojunction bipolar transistor device, and such adevice results in a minimum in zero offset voltage in the measurement ofthe collector current vs. the collector—emitter voltage as a function ofthe stepped voltage bias of the base-emitter junction, which isdesirable to improve the power added efficiency of NPN heterojunctionbipolar transistor 0500. For robust and high power, one would like thecollector bandgap energy to be as large as possible. If the emittermaterial, the base material, and the collector material are alldissimilar, the device would be called an asymmetric doubleheterojunction bipolar transistor device.

The NPN heterojunction transistor can promote efficient transport whenthere is zero or near zero or the smallest conduction band offsetbetween emitter-base-collector. In an NPN device, the electrons are keycarrier that makes up the collector current, and the base-emitterjunction controls this electron current. The base alignment of theconduction band offset can be desirable in some examples. Largeconduction band energy offsets or discontinuities at the emitter-base orcollector-base junctions can hinder electron transport.

FIG. 6 shows a flat band energy diagram through a PNP heterojunctionbipolar transistor 0600 for optimized transport. The figure shows theline-up of conduction band edge 0604 and valence band edge 0605 throughPNP heterojunction transistor 0600. There is a zero valence band or nearzero valence band edge offset ΔE_(V) from P emitter material 0601 to Nbase material 0602 to P collector 0603. The emitter-base conduction bandoffset is represented by ΔE_(CE) and the base-collector conduction bandoffset is ΔE_(CC). In physical situations it is desirable to have thesmallest valence band offset that is possible from P emitter material0601 to N base material 0602 to P collector 0603. Here the bandgapenergy of P emitter material 0601 is larger than the band gap energy ofN base material 0602, where there is a large conduction band offsetΔE_(CE) between P emitter material 0601 to N base material 0602, and thejunction is a heterojunction. The bandgap energy P collector material0603 can be less than, equal to, or greater than the bandgap energy of Nbase material 0602. Generally the bandgap energy of P collector material0603 should be equal to, or greater than the bandgap energy of N-basematerial 0602. The greater the bandgap energy of P collector material0603 the better the breakdown voltage of PNP heterojunction bipolartransistor 0600. This is generally desirable for high power and robustdevices. If P collector material 0603 is the same material as N basematerial 0602, there is a homojunction at the base-collector junctionand a heterojunction at the emitter-base junction, and such a device iscalled a single heterojunction bipolar transistor device. If P emittermaterial 0601 and P collector material 0603 are the same, then thedevice is called a symmetric double heterojunction bipolar transistordevice, and such a device results in a minimum in zero offset voltage inthe measurement of the collector current vs. the collector—emittervoltage as a function of the stepped voltage bias of the base-emitterjunction, which is desirable to improve the power added efficiency ofPNP heterojunction bipolar transistor 0600. For robust and high power,one would like the collector bandgap energy to be as large as possible.If the emitter material, the base material, and the collector materialare all dissimilar the device would be called an asymmetric doubleheterojunction bipolar transistor device.

A PNP heterojunction transistor can promote efficient transport whenthere is zero or near zero valence band offset betweenemitter-base-collector. In a PNP device, the holes are key carriers thatmake up the collector current, and the base controls this hole current.The base alignment of the valence band offset can be desirable in someexamples. Valence band energy discontinuities at the emitter-base orcollector-base junctions can hinder hole transport.

The relationship of the conduction and valence band offsets for manysemiconductors is well studied, and numerous values of the conductionband offsets ΔE_(C) and valence band offsets ΔE_(V) between dissimilarsemiconductors (heterojunction) have been published in the literature.

Unlike homojunction bipolar transistors, heterojunction bipolartransistors (HBTs) allow for a higher base doping density (>1×10¹⁹ cm⁻³)thus reducing the base resistance and maintaining current gain. For NPNHBTs, higher base doping density can occur as a result of the largevalence band offset at the emitter-base junction. For PNP HBTs, higherbase doping density can occur as a result of the large conduction bandoffset at the emitter-base junction.

Typically, one would like to have the highest doping density that ispossible in the base. Typically the highest levels of base doping aregreater than 1×10¹⁹ cm⁻³. High doping levels are typically greater than1×10¹⁸ cm⁻³ range and typically low doping levels are in the 1×10¹⁶ cm⁻³to 5×10¹⁷ cm⁻³ range. The high doping density in the base causes areduction in the base sheet resistance thus allowing the transistor tohave larger F_(max) (e.g., the maximum frequency to get power gain outof the transistor). Also, by having high base doping one can reduce thethickness of the base and increase the F_(t) (e.g., the transitfrequency, time for carrier to go across base region). The relationshipbetween transit frequency F_(t) and the maximum oscillation frequencyF_(max) is as follows for an HBT: F_(max)=(F_(t)/8πR_(B)C_(CB))^(1/2).The transit frequency F_(t) is basically inverse of the time for theelectron to traverse the emitter, base and collector. The parametersR_(B) and C_(CB) refer to the base sheet resistance and the capacitanceof the collector-base junction. The parameter F_(max) is the unity powergain frequency and indicates the maximum frequency with power gain froma device.

The reason why heterojunction bipolar transistors (HBTs) can beadvantageous is that heterojunction bipolar transistors (HBTs) allow fora higher base doping density (>1×10¹⁹ cm⁻³) thus reducing the baseresistance and maintaining current gain. For various choices of theemitter and base materials, it is possible to obtain large valence bandoffset ΔE_(V). This large valence band offset ΔE_(V) prevents the backinjection of minority carriers into the emitter, thus keeping the gainof the HBT high (no degradation of the gain with high doping of the basematerial).

Additionally, in some examples for the base-collector junction, thebase-collector breakdown voltage is set by the energy bandgap of thecollector material. Typically, one would like to have a low energybandgap base material (typically these are relevant semiconductors withbandgaps less than 0.75 eV, like GeSn, Ge, InGaAs, GaAsSb) because thatsets the turn-on voltage of the base-emitter junction or the onset oftransistor action. However, in a homojunction (base and collectormaterials are the same), a low energy bandgap at the collector canresult in a low base-collector breakdown voltage. Thus a large potentialdifference between the base and the collector junction could allow thetransistor to have a low breakdown voltage which causes the transistorto be easily damaged thus hurting ruggedness. In a heterojunctionbipolar transistor, it is possible to combine a low energy bandgap baseregion with a large energy bandgap collector region thus allowing for alarge breakdown voltage. Heterojunctions transistor can be optimizedutilizing optimized material for the emitter, base, and collector.Heterojunction bipolar transistors can optimize the emitter basecollector regions to make high performance and high power transistors.

The characteristics and properties of the base and the base-emitterjunction and the base-collector junction are the dominant factors thatdetermine the electronic properties of bipolar and heterojunctionbipolar transistors. Thus the utilization of a new type of base materiallike GeSn in these transistors allows for the development of vastlyimproved transistors for high speed and power efficient operation.

GeSn is a useful material for use as the base material for bipolartransistors because it can become a direct gap material at higher Sn %,which makes it a useful material for light emission. Thus GeSn in bulkform or GeSn quantum wells (QW) or GeSn quantum wires, or GeSn quantumdots (QD) can act as the active region for light emission in devicessuch as a light emitter, laser or transistor laser.

Lasers are devices that can produce intense narrowly divergent,substantially single wavelength (monochromatic), coherent light. Laserlight of different wavelengths can be advantageously applied in manyfields, including biological, medical, military, space, industrial,commercial, computer, wireless devices, and telecommunications.

Semiconductor lasers may utilize an active region, which may be formedwith a homojunction (using similar materials), single or doubleheterojunction (using dissimilar materials), or with a quantum well(“QW”), quantum dot (“QD”), quantum wire, or quantum cascade region. Theenergy transitions can occur from interband or inter-sub-band electronicstates. The quantum well, quantum dot, or quantum wire structure may beformed when a low energy bandgap semiconductor material is typicallysurrounded or confined by a larger bandgap semiconductor materials.Additionally, these quantum confined heterostructures can be type I,type II, or type III (broken energy alignment). The fundamentalwavelength that characterizes quantum well (QW) or quantum dot (QD) isdetermined primarily by the thickness, composition, and material of thequantum well.

In order for lasing to occur, a laser device typically has a resonantcavity and a gain medium to create population inversion. In some highlyefficient semiconductor laser examples, population inversion generallyoccurs with the injection of electrical carriers into the active region,and the resonant cavity is typically formed by a pair of mirrors thatsurround the gain medium. The method of injection of carriers can bedivided into electrical injection of carriers and optical pumping forinjection of carriers. Electrical injection is generally performed by anelectrical current or voltage biasing of the laser and forms the basisof the electrical injection laser. Optical pumping typically usesincident radiation that allows the formation of electrons and holes inthe laser. Additionally, these methods can be operated in a continuouswave (CW) pulsed, synchronous, or asynchronous modes.

FIG. 7 shows a general configuration of a PN junction laser or injectionlaser diode 0700, with a quantum well or quantum dot 0703 active region.This PN junction device operates on the principle of minority injectionof carriers (electrons and holes, the I_(diode) current) into the activeregion and waveguide 0705. The P⁺ cladding 0701 region may serve as theinjection of holes. N⁺ cladding 0706 may serve as the injection ofelectrons. It can be possible when a low bandgap material is placedinside a larger energy bandgap, like the optical confinement layers OCL0702 and OCL 0704, the formation of QW or QD 0703 can be formed. TheseQW or QD 0703 can serve as the active region for the collection of bothelectrons and holes and produces the inverted population necessary forlaser operation. The wide bandgap P⁺ cladding 0701 and N⁺ cladding 0706semiconductors provide for the optical confinement because their indexof refraction is generally lower than that of the optical confinementmaterials OCL 0702 and OCL 0704. The cladding layers also providefunneling of the electrical carriers to the QW or QD 0703 regions. Light0707 can be produced by recombination of carriers in the QW or QD.Additionally, there are the optical confinement layers (OCL) which serveas the barrier to the QW or QD region thus providing for the quantumconfinement, and also serves as the waveguide material. The OCL layersgenerally have bandgap energies between that of the QW or QD and thewide bandgap energy cladding layers. The combination of the QW or QD0703 and the OCL 0702 and OCL 0704 form the active region and waveguide0705 of the laser structure.

FIG. 8 shows an exemplary flat band energy diagram showing theconduction band edge 0808 and the valence band edge 0809 of a separateconfinement heterostructure (SCH) laser 0800 with an active QW or QD0803 region. The OCL1 0802 and OCL2 0804 form the barrier layers to theQW or QD 0803 which allows for quantum confinement and can be of thetype I heterojunction band alignment. Such a structure providesefficient recombination of the carriers and good optical confinement ofthe light produced from the recombination of the carriers. The P⁺cladding large bandgap 0801 and the N⁺ cladding large bandgap 0806provides for minority carrier injection and funnels the carriers intothe waveguide region ultimately recombining in the QW or QD 0803 activeregion. The cladding layers form the boundary for the waveguide 0807with the OCL1 0802 and OCL2 0804 regions thus providing for efficientconfinement of light. In this design the P⁺ cladding large bandgap 0801and N⁺ cladding large bandgap 0806 have the largest bandgap energies inthe device structure. Next the OCL1 and OCL2 layers have the nextlargest bandgap energies. Finally the QW or QD 0803 materials have thesmallest bandgap energies. In this design the P⁺ cladding large bandgap0801 and OCL1 0802 has a type I heterojunction alignment, and in thisdesign the N⁺ cladding large bandgap 0806 and OCL2 0804 also has a typeI heterojunction alignment. Various configuration of the SCH arepossible for optimization of the laser. Note that though the figureshows only one QW or QD region, multiple QW or QD regions can be used,if higher efficiencies are wanted. Typically such a structure isinserted into a resonant cavity for the light amplification that isrequired for laser operation.

Two common types of semiconductor lasers: (1) in-plane, also known asedge emitting or Fabry Perot lasers (also includes distributed feedbacklasers); and (2) surface emitting also known as vertical cavity surfaceemitting lasers (“VCSELs”). Edge emitting lasers emit light from theedge of the semiconductor wafer whereas VCSELs emits light from thesurface of the laser. In addition, for the edge emitter the resonantcavity is typically formed with cleaved mirrors at each end of theactive region.

FIG. 9 is a perspective schematic of a typical edge emitting injectiondiode laser or in-plane semiconductor laser 0901. The edge emittinglaser 0901 can comprise a substrate 0910 with an active region 0906disposed between a P-type cladding layer 0904 and an N-type claddinglayer 0905. Cleaved facets on the front 0908 and on the back 0909 of thelaser typically form a resonant optical cavity. The order of the layersmay not be restricted as described above. To activate the laser, a biascurrent can be applied to top 0902 and bottom 0903 metal contacts. Uponapplication of the bias to the laser, light of a wavelength λ 0907 istypically emitted from the edge of the laser. An exemplary edge emittingsemiconductor laser may include a GeSn quantum well active regionmaterial 0906 with GaAs barriers for near-infrared (IR) light emission0907, or their equivalents. The QW active region of the structure istypically capable of emitting a designed center wavelength over a widerange of possible wavelengths depending on a number of device designparameters including but not limited to the thickness and composition ofthe layer materials. Being able to tune light over the wide range ofwavelengths could be useful for a variety of applications.

The design and fabrication of this type of edge emitting laser structuremay utilize consideration of the material properties of each layerwithin the structure, including energy band structure and bandalignments, electronic transport properties, optical properties, systemsdesign, and the like. An edge emitting laser such as the exemplary onedescribed above may satisfactorily be wavelength tuned in the mannerpreviously described.

It may also be possible to omit the top metal 0902 and the bottom metal0903 and optically pump the edge emitter from the top, bottom or sidewith another laser that may have an emission wavelength shorter than theedge emitter 0901. This may simplify the process because metallizationof the laser 0901 can be avoided.

The second type of semiconductor laser, VCSELs, emits light normal tothe surface of the semiconductor wafer. The resonant optical cavity of aVCSEL can be formed with two sets of distributed Bragg reflector (DBR)mirrors located at the top and bottom of the laser, with the activeregion (which can be a quantum well, quantum dot, or quantum wireregion), sandwiched between the two Bragg reflectors. Note thedesignation of N DBR means that the DBR is doped N-type.

FIG. 10 is an exemplary schematic of the side view of a vertical cavitysurface emitting laser (“VCSEL”) 1001. A substrate 1006 can havedeposited layers of P-type distributed Bragg reflectors (“DBR”) material1003, and N-type distributed Bragg reflector material 1005. An activeregion 1004 is inserted in the optical confinement layer 1009 thensandwiched between the P DBR 1003 and the N DBR 1005 structures. Metalcontacts 1002, 1007 are provided for applying a bias to the laser. TheP-type DBRs 1003 and N-type DBRs 1005 form the resonant optical cavity.The order of the layers is not restricted as described above. Uponapplication of a current bias to the laser, light 1008 is typicallyemitted from the surface of the laser, which can be the bottom or top ofthe laser. A VCSEL laser 1001 such as the exemplary one described abovecan allow laser emission from the surface of the structure rather thanfrom the side or edge, as in the edge emitting laser of FIG. 9. Thoughthe diagram of an example of a VCSEL shows the light is coming out ofthe bottom, but could be designed so that light comes out of the top.

It can also be possible to omit the top metal 1002 and the bottom metal1007 and optically pump the VCSEL 1001 from the top or bottom withanother laser that can have an emission wavelength shorter than theVCSEL 1001. This may simplify the process because metallization of thelaser 1001 can be avoided.

These lasers can be called diode lasers or injection diode lasers andare two terminal devices. The HBT are three terminal devices. It ispossible to combine both structures to form the light emittingheterojunction bipolar transistor which can act as a three terminaldevice, but also can emit light. Such a device would allow forintegrated circuit designs that could transmit data optically and act ashigh speed switching transistors, all in a single device. Because thelight emitting transistor is a three terminal device, the extra terminalallows the biasing of the base collector junction to quickly collect thecharge carriers, and thereby out performing laser diodes and/or twoterminal devices.

For both the edge emitting laser 0901 and the VCSEL 1001, the inputcontrol to the lasers can be a current bias, voltage bias, or opticalpump techniques as described. Furthermore, both electrical injection andoptical pumping can be operated in continuous wave (CW), pulsed,synchronous, or asynchronous modes of operation.

The light emitting transistor laser could comprise a bipolar transistorwith a direct gap quantum well, quantum dot, or quantum wire inserted inthe base/barrier region. The quantum well, quantum dot or quantum wireforms the collection region (active region) for electrons and holes torecombine to generate light.

In the following figures or tables, N⁺ refers to high N-type doping, N⁻refers to moderate N-type doping, P⁺ refers to high P-type doping, P⁻refers to moderate N-type doping, and UID refers to unintentionaldoping.

FIG. 11 shows a simple diagram of an NPN transistor laser 1100 showingthe three terminal device configuration with the correspondingemitter-base-collector designation, with a quantum well or quantum dotactive 1103 inserted into P base/barrier 1102 and 1104. Here emitter1101 and the collector 1105 can form the cladding regions for opticalconfinement of the light 1107 produced by recombination of the carriersin the quantum active region. The P base forms the barrier region forthe quantum well, quantum dot, or quantum wire and also the waveguidematerial. Because the transistor laser acts as a transistor and a laser,the emitter-base-collector needs to have dual functions for theelectrical and optical. For an NPN HBT laser the emitter has to injectelectrons into the base and also form the cladding layer for lightconfinement as designated by N emitter/cladding 1101. The base regionshould be heavily P-type doped and forms the barrier to the QW or QD1103 material and thus is designated by the P base/barrier 1102 and Pbase/barrier 1104. The P base/barrier 1102 and 1104 and the QW or QD1103 form the active region and the waveguide 1106 of the NPN transistorlaser 1100. To additionally form for example, a light edge emittingtransistor laser, a resonant cavity is typically formed by cleavingmirrors at the front and back facets of the crystal to optically amplifythe photon population.

FIG. 12 shows a cross-sectional device depiction of an NPN edge emittingtransistor laser 1200. The device comprises an N⁺ substrate 1201 as thestarting point. A heavily doped N⁺ sub-collector 1202 is grown on the N⁺substrate 1201. Then a dual purpose lightly doped N− collector/bottomcladding 1203 layer is formed. Then a P⁺ base/barrier/opticalconfinement layer 1204 is grown and this also starts the formation ofthe waveguide region for the confinement of the light. Next the QW or QD1205 active region is grown, and then on top of this layer is the P⁺base/barrier/optical confinement layer 1206 which finishes the waveguideportion of the device. On top of this layer is then grown the N⁻emitter/upper cladding 1207 which provides the cladding finishing up thedevice. Metal contacts are placed at the N⁺ emitter contact 1208 via thetop metal 1209 contact, the P⁺ base/barrier/optical confinement layer1206 via the base metal 1210, and the N⁺ substrate 1201 via the bottommetal 1211. The resonant cavity is formed by the front cleaved crystalfacets 1214 and the back cleaved crystal facets 1213. Light 1212 isemitted from the front of the device. The device is biased in a standardNPN transistor configuration, but because of the inclusion of the QW orQD 1205, P⁺ base/barrier/optical confinement layer 1204 and 1206 and theN− collector/bottom cladding 1203 and N⁻ emitter/upper cladding 1207,the device will form a light emitting or laser transistor when properlydesigned. A typical way of making this structure is to use epitaxialtechniques to grow the basic layered structure then use standard deviceprocessing techniques to fabricate the full device. The laser includesquantum well, quantum dot, or quantum wire inserted into a base regionof the heterojunction bipolar transistor. The laser can require aresonant cavity to get optical gain, and typically this can be formedfrom the front and back cleaved facets of the semiconductor crystalwafer.

Vertical emission of light normal to the surface of the semiconductorwafer is also a useful configuration for transistor lasers. The resonantoptical cavity of a vertical transistor laser can be formed with twosets of distributed Bragg reflector (DBR) mirrors located at the top andbottom of the laser, with the active region (which can be a quantumwell, quantum dot, or quantum wire region), sandwiched between the twoBragg reflectors.

FIG. 13 shows a cross-sectional device depiction of an example of apossible configuration of a NPN VCSEL transistor laser 1300, where thelight is coming out of the top, but also could be designed so that lightcomes out of the bottom. The device can be grown on an N⁺ conductingsubstrate 1301, with the growth of a bottom N⁺ DBR 1302 stack whichforms the bottom mirror of the device. N⁻ collector 1303 is then grownon the bottom mirror. P⁺ base 1304 is then grown on the N⁻ collector1303, and also forms the barrier for QW or QD 1305 active region. QW orQD 1305 is deposited on P⁺ base 1304, and then P⁺ base 1306 is depositedon QW or QD 1305, finalizing the barrier material to the active regionfor quantum confinement effects. N⁻ emitter 1307 is then grown on P⁺base 1306. Then an N⁺ contact 1308 is deposited on N⁻ emitter 1307.Finally a dielectric mirror stack D DBR 1309 is deposited on N⁺ contact1308. VCSEL transistor laser 1300 is processed using standard techniquesof mesa etch and metallization to form the final structure, where metalcontacts emitter metal/aperture 1311 is deposited on N⁺ contact 1308.Emitter metal 1311 forms an aperture for light out 1310. Base metal 1312is deposited on P⁺ base 1306, and bottom metal 1313 is deposited on N⁺substrate 1301. NPN VCSEL transistor laser 1300 is operated as astandard bipolar device.

GeSn is alloy semiconductor of the constituent semiconductors germanium(Ge, which is an indirect semiconductor, with an energy bandgap of 0.66eV) and alpha tin or cubic tin (Sn which is zero energy gap directsemiconductor). GeSn can be an indirect or direct energy bandgapsemiconductor depending on the alloy composition. A direct gapsemiconductor has its conduction band minimum energy and valence bandenergy maximum occur at the same crystal momentum (k-space). If thelocation of the conduction band energy minimum and the valence bandenergy maximum occurs at different crystal momentum (or differentlocation in k-space), it is an indirect semiconductor. Direct gapsemiconductors are highly efficient for radiative recombination ofelectrons and holes, thus most light emitting devices are fabricatedfrom direct gap semiconductors.

GeSn semiconductors have been grown epitaxially by metalorganic chemicalvapor deposition (MOCVD), molecular beam epitaxy (MBE), ion implantationfollow by anneal, by pulse laser deposition (laser ablation), but,additionally, liquid phase epitaxy, vapor phase epitaxy and variousother epitaxial growth techniques can be used to grow the GeSn materialdescribed herein. The GeSn layers have been grown up to 20% Sn content.Both N-type and P-type doping has been achieved in GeSn layers.

FIG. 14 shows the energy band structure diagrams for the semiconductorsGe 1401 and Sn 1402. The vertical axis is the Energy with units of (eV)and the horizontal axis is the Wavevector “k”. The energy band structurediagrams depict the available or unavailable (forbidden gap) energylevels for the charge carriers in the semiconductor. The plot showsenergy on the vertical scale and the crystal momentum direction on thehorizontal scale. There are significant crystal momentum points “X,” “Γ”(Brillouin zone center) and “L.” Ge 1401 is an indirect gapsemiconductor because the conduction band minima is at the “L” point andthe valence band maximum is at the “Γ” point. Sn 1402 is a semimetal orzero direct gap semiconductors with the conduction band minima andvalence band maxima at the “Γ” point. As Sn is added to Ge, theconduction band energy at the “Γ” point moves down faster than theconduction band energy at the “L” point, and thus, turning GeSn at somealloy composition turns into a direct gap semiconductor.

The direct to indirect transition can occur about 7% Sn in GeSn, but canbe observed up to 11% Sn. The energy bandgap at 7% Sn is about 0.585 eV,this corresponds to a wavelength of about 2370 nm. Ge is a group IVsemiconductor and though it is an indirect semiconductor, it has someproperties that are advantageous. Ge 1401 has a local minimum at the “Γ”point of the conduction band. The lowest energy point in the Ge 1401conduction band is at the “L” point and is only 0.14 eV lower than the“Γ” point a room temperature. Various methods can be used to lower thegamma point below the “L” point such as introducing biaxial tensilestrain or heavily N-type doping the Ge. However, by adding Sn to Ge, itis possible to lower the bandgap but also form a direct gapsemiconductor. In addition, one could employ both tensile strain andadding Sn to Ge to make a direct gap semiconductor. The othermethodology to make Ge into a direct gap semiconductor is by applyingtensile strain on the Ge of greater than 1.4%. In some embodiments, thetensile strain can be a biaxial tensile strain.

In some examples, heterojunction bipolar transistors (HBT) can be adesirable device for greater power handling capability, higher powerefficiency, and lower signal distortion. The fabrication of a GeSn basedHBT structure enables a new transistor technology that can significantlyoutperform SiGe, GaAs, InP, and GaN transistors in high-power,high-frequency applications. The new HBT semiconductor structuredescribed herein exhibits a large valence band discontinuity between theemitter and base; has a low energy bandgap base (the term low energybandgap base typically refers to the relevant semiconductors withbandgaps less than 0.75 eV, like GeSn, Ge, InGaAs, and GaAsSb); and asecond (double) heterojunction can be inserted between the base andcollector with a good breakdown electric field. These attributespositively can impact several key device parameters such ascollector-emitter breakdown voltage, DC current gain, and power gaincutoff frequency (F_(max)). The low bandgap GeSn base can significantlydecrease transistor turn-on voltage and thereby increase the power addedefficiency of the device.

FIG. 15 shows a graph of the collector current density J_(C) vs. theturn-on voltages (V_(BE)) of various HBT material systems. The figureshows a plot of the collector current density J_(C) (A/cm²) verticalscale versus base-emitter voltage V_(BE) (V), horizontal scale. Theplotted characteristics (ideal) for several different heterojunctionbipolar transistor (HBT) technologies are shown. The GeSn HBT structuredescribed herein has the lowest turn-on voltage 1501 of the technologiesshown of Ge, InP/InGaAs, SiGe, GaAs, and GaN/InGaN.

For NPN heterojunction transistors, it is generally desirable for thebase region to be heavily P-type doped. This allows for the base sheetresistance to be minimized thus allowing for high frequency operation ofthe transistor. The GeSn base can be heavily doped P-type in someembodiments. When the hole concentration as measured by Hall Effect,high doping levels (>1×10¹⁹ cm⁻³) can be achieved in GeSn.

FIG. 16 shows the hole concentration of GeSn films as a function of Sn %as measured by Hall effect. The vertical axis is the hole concentration(cm⁻³) and the horizontal axis is the Sn %. From the hole concentrationdata vs. Sn % of FIG. 16, it is readily seen that GeSn can be P-typedoped at the highest levels of base doping which are greater than 1×10¹⁹cm⁻³. Additionally, GeSn can achieve a large hole mobility (Ge holemobility is about 1800-2000 cm²/Vs, as compared to GaAs hole mobility400 cm²/Vs) which is a precondition for making the base region thin.

By utilizing GeSn in the base of a heterojunction bipolar transistor,one can lower the turn-on voltage because the bandgap of GeSn is lessthan that of Ge, which is less than that of the materials systems GaN,GaAs, Si, InP, GaP, AlAs, and Ge. Additionally, at low Sn content, GeSnhas all the advantages that a Ge base material adds. For an NPNstructure, Ge is desirable for the base region because it can be heavilydoped P-type, it has the highest hole mobility (desirable for reducingthe resistance of the base), also this hole mobility can be increased byapplying tensile or compressive strain, and its conduction bandalignment is favorable with numerous semiconductors. At about 7% Sn andgreater, GeSn goes from an indirect semiconductor to a direct gapsemiconductor, though this indirect to direct transition can occur at Snpercentages up to 11%.

FIG. 17 shows a representative graph showing the GeSn direct bandgapenergy vs. lattice constant. The vertical axis shows the values of theEnergy Bandgap (eV) and the horizontal axis shows the Lattice Constant(A). The dots and the line through the dots represent GeSn direct gapenergy as a function of its lattice constant. Because the data is forthe direct bandgap transition, the first data point is near 7% Sn andthe last is near 20% Sn. Ge has an indirect bandgap energy of 0.66 eV.GeSn is indirect up to about 7% Sn then becomes a direct gapsemiconductor with an energy gap of 0.585 eV and a lattice constant ofabout 5.725 Å, and the bandgap energy typically reduces to about 0.25 eVat 20% Sn with a lattice constant of about 5.835 Å. The emissionwavelengths that can occur in bulk direct gap GeSn range from about 2370nm at 7% Sn to 5540 nm at 20% Sn.

FIG. 18 shows a replot of FIG. 17 showing a representative graph of theGeSn emission wavelength vs. lattice constant. The vertical axis showsthe values of the Wavelength (nm) and the horizontal axis shows theLattice Constant (Å). The dots and the line through the dots representGeSn possible emission wavelength as a function of its lattice constant.For on-chip communications 1000 nm to 3000 nm is acceptable. Fortelecommunications applications, the typical wavelengths used are 1300nm and 1550 nm. These wavelengths can be achieved by using quantum wellor quantum dot GeSn materials. These low dimensional structures like twodimensional “2D” QW, one dimensional “1D” quantum wires, or zerodimensional “0D” quantum dot structures increase the light emissionenergy due to quantum confinement effects.

It should be noted that GeSn is an alloy semiconductor and that Snpercentages can be varied from 0%≤Sn %≤20%.

GeSn materials are useful for quantum confined structures. Quantumconfined structures such as quantum wells (QWs), quantum dots (QDs), andquantum wires structures add a new degree of freedom in making lightemitting materials. Because GeSn can become a direct gap material at 7%to 11% Sn content, it would be useful to have possible wavelengths inthe 1000 to 4200 nm range because this covers the telecommunications andfiber optics networks. One method of taking GeSn bulk material to getenergies that cover this wide range is to use quantum well, quantum dot,or quantum wire technologies, because the light emission is thendependent on quantum confinement or quantum size effects.

QDs form artificial semiconductor atoms with electronic “shells” thatcan be engineered to control their light absorption properties. Besidestheir novel electronic properties, QDs also have interesting materialproperties; their 3-dimensional shape allows greater strain relief atthe QD surfaces than for planar growth. GeSn QDs can be grown on Siwithout creating dislocations. Absorption over broad wavelengths comesfrom an ensemble of QDs that have sizes that vary statistically. Alsobecause GeSn at low Sn content starts as indirect material and thenbecomes a direct gap material at higher contents, it is possible toproduce efficient light emission in QD structures with indirect gapsemiconductors. The limitations of the indirect nature of the bandgapGeSn (Sn %<7%) can be overcome by the formation of low-dimensionalstructures such as quantum dots because this method uses the spread ink-space caused by the quantum confinement to circumvent the indirectbandgap problem of the GeSn. Thus QDs are useful for producing lightemission in direct and indirect gap materials.

The use of GeSn as the active layer of the quantum dot laser hassignificant advantages. The greater than 4% mismatch between GeSn and Siallows for the self-assembly of Ge islands by the Stranski-Krastanovgrowth mode (strained layer epitaxy). This 3-dimensional growth mode isa method of making zero dimensional structures (i.e., QD). For QDs toeffectively provide light emission, the QD material is generally of alower energy bandgap than the barrier material. The relatively low GeSnbandgap energy makes it a desirable starting point for absorption in thenear-IR and mid-IR. It is possible by controlling the size of the Gequantum dots, to change the interband (electron-hole recombination) toallow for energy transitions in the near-IR to mid-IR. The GeSn QD withSi barriers can be of type II band alignment.

FIG. 19 shows that the formation of quantum dot structures are a resultof ability of self-assembled GeSn by the Stranski-Krastanov (SK) orstrained layer epitaxy 1900 method that transitions from two dimensionalplanar growth 1901 to island growth 1902. In SK growth methodology alarger lattice constant semiconductor GeSn film 1903 is grown on asemiconductor with a smaller lattice constant Si 1904. Under properconditions two dimensional planar growth 1901 starts but quicklytransitions to island growth 1902 thus forming the GeSn QD 1905structure. Thus when a larger lattice constant semiconductor is grown ona semiconductor with a smaller lattice constant, the critical thicknessof the larger lattice constant layer is exceeded and QDs can form. Thelattice mismatch between the two layers should be generally greater than2% for dot formation. The lattice mismatch of GeSn is grown on Si with alattice constant of 5.43 Å, starts off at a 4% lattice mismatch for lowSn % and can reach up to 20% Sn content at a 7% lattice mismatch withSi. If SiGe layers are used as the barrier depending on the Ge contentin the SiGe, the lattice mismatch could be significantly reduced.Typically quantum dots are less than 15 nm, but they can range from 1 to100 nm in size. Strained layer epitaxy 1900 is a methodology for growingquantum structures with dissimilar lattice constant materials. After the3D growth of the QD, the QD layer usually has a barrier layer grown ontop to finalize the quantum confinement.

FIG. 20 shows a flat band energy diagram of type II interband GeSn QDwith Si barriers 2000. The GeSn QD 2003 to Si 2002 heterojunction whichcan be of a type II heterojunction band alignment but at higher Sn % canbecome type I alignment. The figure shows a type II interband 2001transition from the conduction band of the Si 2002 to the GeSn QD holelevel 2004. The energy bandgap is indicated in parenthesis below thematerial of interest. These type II interband 2001 transitions allow thepossibility of light emission. Type II energy band alignments allow foremission energy levels that can be the closest to the energy bandgap ofthe bulk semiconductor. Note the size L_(QD) 2005 of the GeSn QD 2003determines the energy difference of the type II interband transition. Bychanging the size L_(QD) one can change the emission wavelength of thestructure.

FIG. 21 shows the possible range of emission wavelengths that areachievable in a type II GeSn quantum dot heterostructure with Sibarriers. The vertical axis is the wavelength (nm). The horizontal axisis the Quantum dot size (nm). This possible data represents the case forlow Sn % GeSn alloys. The graph shows that emission wavelengthsachievable depend on the size of the quantum dot. Note the wavelengthsare in the range of telecommunications. If the Sn % is increased thewavelengths achievable will get longer.

A type I heterostructure band alignment can occur for a GeSn QD if thesubstrate of interest is SiGe, because the addition of Ge to Si barriersalter the Si band structure. The addition of Ge into Si increases thelattice constant, thus SiGe has a larger lattice constant than Si. Thusit is possible to obtain higher Sn % GeSn QD on SiGe because the SiGelattice constant is larger than the Si lattice constant.

FIG. 22 shows type I alignment of the GeSn QD with SiGe barriers 2200.The flat band energy diagram of GeSn QD 2203 with SiGe 2202 barrierswhich can be of type I alignment. The figure shows a type I interband2201 transition from the conduction band of GeSn QD electron level 2204to the GeSn QD hole level 2205. These type I interband 2201 transitionsallow the possibility of light emission. Note the size L_(QD) 2206 ofthe GeSn QD 2203 determines the interband transition energy. Because thearrangement is of a type I heterostructure the emission energies thatcan be achieved are typically higher than in a type II heterostructure.It is straightforward to calculate the range of wavelengths achievablein the GeSn quantum dot with SiGe barriers. Basically the emissionwavelengths that can be achieved for a type I alignment are shorter thanthat of type II QD heterostructures. By utilizing SiGe 2202 one canchange the Ge content of the SiGe thus changing the properties of theSiGe barrier layer to the GeSn QD 2203, thus in this structure theemission wavelengths achievable depends on the QD size L_(QD), the ratioof Ge to Sn in the GeSn 2203 material, and the ratio of Si to Ge in theSiGe 2202 barrier layer. If the Ge content is high enough (greater than50%) in the SiGe barrier it can be possible to grow GeSn in a planargrowth mode thus forming two dimensional growth or QWs.

The formation of QWs are more straightforward because the materials aregrown in a planar structure, and the QW can be coherently strained ornear lattice matched. FIG. 23 shows the methodology of planar growth2301 of the GeSn 2303 QW region on the GaAs 2302 bottom barrier layer.GaAs 2302 bottom barrier layer has a larger lattice constant than theGeSn 2303. But for low Sn % that GeSn 2203 can be grown coherently orpseudomorphic on the GaAs 2302. To finalize the QW layer a GaAs 2304 topbarrier layer is grown on the GeSn 2303 QW layer. Note that the barrierlayer generally has a larger bandgap energy than the QW layer. Thethickness of the QW region and the band alignment to the barriermaterials determine the allowable energy transitions.

FIG. 24 shows a flat band energy band diagram of a type I interband GeSnQW with GaAs barriers 2400. The band alignment of GeSn QW 2403 with GaAs2402 barrier can be of a type I heterojunction. Because the latticeconstant of GaAs and Ge is both about 5.66 Å, one could also use a Gebarrier material to obtain a type I alignment. Because for low Sn %GeSn, the GeSn growth on GaAs or Ge will be compressively strained. Aslong as the GeSn thickness is less than the critical thickness, coherentplanar or pseudomorphic growth can proceed on the GaAs or Ge barriermaterial, thus forming the GeSn QW. The Type I interband transitions arean excellent method for producing the emission of light fromsemiconductor heterostructures. The FIG. 24 shows a type I interband2401 transition from the conduction band GeSn QW electron level 2404 tothe GeSn QW hole level 2405. These type I interband 2401 transitionsallow the possibility of light emission. Note the size L_(QW) 2406 ofthe GeSn QW 2403 determines the type I interband transition energy.Because the arrangement is of a type I heterostructure the emissionenergies that can be achieved are typically higher than in a type IIheterostructure. It is straightforward to calculate the range ofwavelengths achievable in the GeSn quantum dot with low Sn % with GaAsbarriers. Basically the emission wavelengths that can be achieved for atype I alignment are shorter than that of type II QD heterostructures.

For QW of a type I heterostructure the emission energies that can beachieved are typically higher than in a type II heterostructure. Type Iinterband transitions generally result in energy transitions that aregreater than the bulk GeSn transitions. Type II transitions can resultin energy transitions that can be less than the bulk GeSn transitions.Basically the emission wavelengths that can be achieved for a type Ialignment are shorter than that of type II QW heterostructures. As theSn % in GeSn increases the QW wavelengths will get longer.

By utilizing a direct energy gap GeSn bulk material or a GeSn quantumwell or a GeSn quantum dot in the base region of a transistor canachieve a light emitting HBT that can emit light from 1000 nm to 5000nm.

For an NPN HBT utilizing a GeSn base region certain requirements arenecessary to optimize the structure. FIG. 25 shows the general flat bandenergy diagram of an NPN GeSn double heterojunction bipolar transistor.This shows the general configuration of an optimized NPN HBT with GeSnbase 2500 region (energy bandgap is indicated in parenthesis below thematerial of interest) can include an N-type emitter of material 1 E_(G1)2502, with a E_(G1) energy bandgap greater than the GeSn 2501 (has anenergy band gap which is less than 0.66 eV) and forms the P-type baseregion; then an N-type collector material 2 E_(G2) 2503 where the energybandgap energy E_(G2) which can equal or be greater than the material 1E_(G1) 2502. Furthermore, the conduction band offset energies ΔE_(C1)2504 at the emitter base junction and the ΔE_(C2) 2505 at the basecollector junction should be zero or near zero in value. Smallconduction band offsets between the emitter-base junction and thecollector-base junction are desirable for electron transport.Additionally, the valence band offset ΔE_(V1) 2506 at the emitter basejunction should be as large as possible to ensure that there is no backinjection of holes from the GeSn 2501 P-type base to material 1 E_(G1)2502 N-type emitter material.

HBT performance can be improved, in some examples, by grading thecomposition of the base to decrease the energy bandgap gradually throughthe base. The grading of the base energy bandgap can create an electricfield, which causes a reduction in the transit time of the chargedcarriers. The slope of Ge to GeSn compositional grade in the base inthis example can be varied from linear to discontinuous functions. Thegraded GeSn may comprise starting growth of Ge at the emitter thengrading to higher Sn % GeSn at the collector or may comprise startinggrowth of low Sn % GeSn at the emitter then grading to higher Sn % GeSnat the collector.

FIG. 26 shows the general flat band energy diagram of an NPN HBT with agraded Ge—GeSn base 2600 region. Starting next to the N emitter material1 E_(G1) 2602 with a Ge or a low Sn % GeSn layer which is graded tohigher Sn % GeSn and is represented by Ge—GeSn 2601. Such a structureresults in a field enhancement 2608 region. The general configuration ofthis NPN HBT with the graded Ge—GeSn 2601 P-type base region can includean N-type emitter of material 1 E_(G1) 2602, with an E_(G1) energybandgap greater than the Ge—GeSn 2601 energy bandgap which is less than0.66 eV; a Ge—GeSn 2601 P-type base region; then a N-type collectormaterial 2 E_(G2) 2603 where the energy bandgap energy E_(G2) may equalor be greater than that of material 1 E_(G1) 2602. Furthermore, theconduction band offset energies ΔE_(C1) 2604 at the emitter basejunction and the ΔE_(C2) 2605 at the base collector junction should bezero or near zero in value. Having small conduction band offsets betweenthe emitter-base junction and the collector based junction is desirablefor electron transport. Additionally, the valence band offset ΔE_(V1)2606 at the emitter base junction should be as large as possible toensure that there is no back injection of holes from the Ge—GeSn 2601P-type base to material 1 E_(G1) 2602 N− type emitter material.

The importance of the base region of the HBT can be further elucidatedby the following example. A GaAs base HBT has a base thickness of 1000Å, for an equivalent device a GeSn base HBT, the base thickness could behalved to 500 Å with no detrimental but enhanced results. The F_(t) forGeSn HBT would increase because of the thinner base. Because the GeSnbase resistivity (0.0002 ohm-cm) is 10 times less than GaAs resistivity(0.002 ohm-cm), the parameter F_(max) would increase by a factor of(5*F_(t))^(1/2). Thus F_(t) increased because the base thickness washalved, and the base sheet resistance of the GeSn base only increased bya factor of two but it is still 5 times less than the base sheetresistance of the GaAs HBT. Thus GeSn is an excellent material for highfrequency HBT performance.

GeSn base material advantages. At low Sn % comprise similar propertiesto Ge. GeSn has a low bandgap (lower than Ge: the term low energybandgap base typically refers to the relevant semiconductors withbandgaps less than 0.75 eV, like GeSn, Ge, InGaAs, and GaAsSb) whichresults in low turn-on voltage (less than 0.5 V) for the base emitterjunction. GeSn (low Sn %<20%) hole mobility is high like Ge (2000cm²/Vs) as compared to GaAs (400 cm²/Vs) and acceptors can beincorporated to high density (>1×10¹⁹ cm⁻³). GeSn base can be madeultra-thin (much less than 500 Å) while maintaining a low base sheetresistance (P-type base resistivity is about 0.0002 ohm-cm) whichincreases current gain and decreases electron transit time. GeSn can beheavily doped P-type (>1×10¹⁹ cm⁻³). GeSn for low Sn concentration, hasshallow acceptors, so the hole concentration is generally equal to theacceptor doping level and independent of temperature. The low base sheetresistance of GeSn results in a high F_(max). The surface recombinationvelocity is low for P-type Ge and GeSn. GeSn can be made to become adirect gap semiconductor at compositions in the range from 7% to 11% Sncontent, thus useful as a light emitting semiconductor material.

A light emitting heterojunction bipolar transistor can be formed byplacing a GeSn quantum well or quantum dot in the base region of aheterojunction transistor. This is a methodology for the formation ofthe transistor laser.

FIG. 27 shows the resulting flat band energy diagram of an NPN HBT withGeSn QW or QD inserted in the P-type base region 2700 for the initialformation of a light emitting transistor. Material 3 E_(G3) 2703 is theP-type base region and forms the barrier to the GeSn QW or QD 2707 toget quantum confinement and also can serve as the waveguide material.The material 3 E_(G3) 2703 bandgap energy should be greater than thebandgap energy of the bulk GeSn material which is less than equal to0.66 eV. The N-type emitter material 1 E_(G1) 2701 and N-type collectormaterial 2 E_(G2) 2702 can serve as the cladding layers of thetransistor laser. The general configuration of an optimized NPN HBT caninclude an N-type emitter of material 1 E_(G1) 2701, with an E_(G1)energy bandgap greater than material 3 E_(G3) 2703 P-type base region.The N-type collector material 2 E_(G2) 2702 should have a bandgap energyE_(G2) equals to the material 1 E_(G1) 2701 or can be much larger. Theconduction band offset energies ΔE_(C1) 2704 at the emitter basejunction and the ΔE_(C2) 2705 at the base collector junction need not benear zero because a type I alignment assists in funneling the carriersinto the waveguide and to the GeSn QW or QD 2707. Additionally, thevalence band offset ΔE_(V1) 2706 at the emitter base junction should beas large as possible to ensure that there is no back injection of holesfrom the P-type base to material 1 E_(G1) 2701 N-type emitter material.

Additional variations could include grading of the quantum region in thebase material of such a device. FIG. 28 shows a slight variation to FIG.27 where the flat band energy diagram of NPN HBT has a GeSn QW or QD2807 inserted in the base region where the barrier layer has been graded2808 from the P base material 3 2703.

Exemplary Configurations: Note these are exemplary heterojunctionbipolar transistor and/or transistor laser configurations and are usedto illustrate the purposes and uses of the various configurations. Invarious embodiments, the GeSn base region can be replaced by a graded Geto GeSn base region.

Exemplary Configuration 1: An NPN structure of a GaAs Emitter—GeSnBase—GaAs Collector symmetric double heterojunction transistor.Typically GaAs HBTs have been the standard of the industry. The deviceelucidated in this example can include a symmetric double heterojunctionGaAs—GeSn—GaAs HBT device. This device can have desirable basecharacteristics with a low voltage base turn-on (<0.66 eV depending onSn %, at 20% Sn % the bandgap energy can be 0.25 eV) region and asymmetric double heterojunction thus eliminating the offset voltage inthe transistor output characteristic that reduces power addedefficiency.

FIG. 29 illustrates an exemplary flat band energy diagram of a GaAsEmitter—GeSn Base—GaAs Collector NPN symmetric double HBT 2900. Note thenear zero conduction band offset ΔE_(C)˜0.05 eV 2901 with a largevalence band offset ΔE_(V)˜0.72 eV 2902. Near zero conduction bandoffsets are generally less than 0.1 eV. This unique arrangement ofmaterials combines the high transconductance of heterojunction bipolartransistor (HBT) technology, and a desirable emitter-base heterojunction(wide bandgap GaAs 2904 emitter on a narrow bandgap high conductivity P⁺GeSn 2903 base). The large valence band discontinuity between the GaAsemitter and GeSn base allows one to lightly N dope the GaAs emitter,while heavily doping P base GeSn 2903. The large valence banddiscontinuity prevents the back injection of holes into the emitter thuspreventing the degradation in the gain or beta of the transistor. TheGaAs 2905 collector provides a reasonable base collector breakdownbecause the band gap energy of the GaAs is 1.42 eV.

This unique arrangement of materials combines the high transconductanceof heterojunction bipolar transistor (HBT) technology, with thebreakdown voltage (using a GaAs collector), and a desirable emitter-baseheterojunction (wide bandgap GaAs emitter on a narrow bandgap highconductivity P-type GeSn base). The combination of a low bandgap (<0.66eV depending on Sn %) GeSn base coupled with a wide bandgap GaAs (can beabout 1.42 eV) collector can be used for high speed power applications.This symmetric double heterojunction bipolar transistor device resultsin a minimum in the zero offset voltage in the measurement of thecollector current vs. the collector-emitter voltage as a function of thestepped voltage bias of the base-emitter junction, which is desirablefor improving the power added efficiency of the NPN heterojunctionbipolar transistors. The use of efficient GaAs—GeSn—GaAs transistors cansignificantly enhance battery life while also enabling operation at highfrequency response, which can be desirable when used as RF poweramplifiers for wireless devices or cellular phone applications.

FIG. 30 shows an exemplary cross-sectional device depiction ofembodiment of an NPN GaAs—GeSn—GaAs symmetric double HBT 3000. Thisdevice depiction shows the standard HBT in a mesa configuration. One cangrow this device epitaxially with a variety of techniques like MBE,MOCVD, etc. One starts with a high quality single crystalsemi-insulating GaAs substrate 3001. An N⁺ GaAs sub-collector 3002 isgrown first, followed by the N⁻ GaAs collector 3003, then the GeSn Base3004 (which has been grown by MBE, MOCVD, PLD, etc.). An N⁻ GaAs emitter3005 is grown on the P-type base, followed by the N⁺ GaAs contact layer3006. Contact is made to the device through the emitter metal 3007, thebase metal 3008, and the collector metal 3009.

Additionally, the base can be compositionally graded from Ge—GeSn tohave field enhancement of the carriers. FIG. 31 illustrates an exemplaryflat band energy diagram of an NPN structure of a GaAs Emitter—gradedGe—GeSn Base—GaAs Collector double HBT 3100. FIG. 31 is a variation onFIG. 29, by including the compositionally graded Ge—GeSn 3101 P-typebase region; this structure creates an electric field that acceleratesthe electrons across the base to the collector, thus creating the fieldenhancement 3102 region. The compositionally graded Ge—GeSn 3101 layercan comprise at the emitter a Ge or a low Sn % GeSn layer which isgraded to higher Sn % GeSn at the collector interface. The compositionalgrading range can go from Ge at the emitter to GeSn at variouscompositions up to 20%.

Table 1 shows an exemplary structure that could be grown for an NPNstructure of a GaAs Emitter—GeSn Base—GaAs Collector doubleheterojunction transistor. Note the table shows a GeSn base region or acompositionally graded Ge—GeSn base region, either which can be used inthe structure.

TABLE 1 Exemplary Epitaxial Structure of NPN GaAs-GeSn-GaAs HBT. LayerLayer Name Description Comment 1 N⁺ Cap ~1000 Å InGaAs (Te-doped >10¹⁹cm⁻³) Te = tellurium InGaAs layer is fully relaxed 2 N⁻ Emitter Cap~1500 Å GaAs (Si-doped~5 × 10¹⁸ cm⁻³) Si = silicon 3 N⁻ Emitter ~500 ÅGaAs (Si-doped~3 × 10¹⁷ cm⁻³) 4 P⁺ Base ~500 Å GeSn (B-doped >10¹⁹ cm⁻³)Or graded Ge-GeSn 0 ≤ Sn % ≤ 20% Thickness range 100 Å-5000 Å 5 N⁻Collector ~10000 Å GaAs (Si-doped~1 × 10¹⁶ cm⁻³) 6 N⁺ Sub-Collector~5000 Å GaAs (Si-doped~5 × 10¹⁸ cm⁻³) 7 High Purity Buffer ~500 Å GaAs(undoped) UID 8 GaAs semi-insulating substrate Note the compositionallygraded Ge-GeSn layer can comprise at the emitter interface a Ge or a lowSn % GeSn layer which is then graded to higher Sn % GeSn at thecollector interface. The grading range can go from Ge at the emitter toGeSn at various compositions up to 20%.

Exemplary GaAs advantages: The large valence band offset between GaAsemitter and

GeSn base can stop back injection of holes into the emitter. This allowsfor low N-type doping of the emitter and high P-type doping of the base,thus lowering base emitter capacitance while still achieving sizablecurrent gain. GeSn is near lattice matched to GaAs (˜5.65 Å), whichenables dislocation free growth.

The GaAs—GeSn emitter base junction has a large valence (˜0.72 eV whichis larger than the GeSn bandgap). This eliminates the back injection ofholes to the emitter from the base, which reduces the gain of thetransistor. Additionally, the base is doped heavily P-type (typically>1×10¹⁹ cm⁻³), with such high doping of the base, the emitter valenceband offset blocks the holes even though the base doping is much higherthan the N-type emitter doping (˜low 10¹⁷ cm⁻³). Furthermore, becauseGeSn has a low resistivity of 0.0002 ohm-cm, one can decrease thethickness of the base significantly, while still moderately increasingthe base sheet resistance value. The frequency response of the device isrelated to the F_(t) and F_(max). The relationship between transitfrequency F_(t) and the maximum oscillation frequency F_(max) is asfollows for an HBT: F_(max)=(F_(t)/8πR_(B)C_(cB))^(1/2). The transitfrequency F_(t) is basically the inverse of the time for the electron totraverse the emitter, base, and collector. The parameters R_(B) andC_(CB) refer to the base sheet resistance and the capacitance of thecollector base junction. The parameter F_(max) is the unity power gainfrequency and indicates the maximum frequency with power gain from adevice. The transit frequency can be further improved by having a highersaturation velocity for the collector.

It should be noted that there are many types of N-type and P-typedopants. For standard III-V semiconductors like GaAs, InP, InGaAs,InGaP, the N-type dopants are Si, Ge, Sn, Pb, S, Se, Te. The P-typedopants for standard semiconductors are C, Zn, Be, Mg. Common dopantsfor group IV semiconductors like GeSn, Ge, Si, SiGe, GeSiSn for N− typedopants are P, As, Sb. The P-type dopants are B, Al, Ga.

Also designations such as N⁺ reference highly N-type doped material andN⁻ lightly doped N-type material. Also designations such as P⁺ referencehighly P-type doped material and P⁻ lightly doped P-type material.Unintentionally doped material can be denoted as UID.

In some examples, to fabricate a light emitting bipolar transistor mayrequire an insertion into the Ge base region (or GaAs base), a GeSnquantum dot or quantum well. FIG. 32 shows an exemplary flat band energydiagram of an NPN transistor laser structure with a GeSn QW or QD activeregion in a Ge P⁺ base/barrier material 3200. The Ge 3201 and 3202 formsthe P⁺ base and also acts as a barrier layer for quantum confine theelectrons and holes in the GeSn QW or QD 3203. QWs are formed by havinga large energy bandgap material surrounded by a low energy bandgapmaterial which results in two dimensional electron confinement. For a QDthe growth of a large lattice constant material on a smaller latticeconstant material results in strained layer epitaxy allowing theself-assembled three dimensional island growth. Typical thickness ofquantum wells can be about 10 nm but they can range from 1 nm to 50 nmdepending on the wavelength of interest that needs to be produced.Typical quantum dot diameters are in the range of 1 nm-20 nm, but aredependent on the wavelength of light that needs to be emitted. The GeSnQW or QD 3203 inserted into a Ge 3201 & 3202 base/barrier serves for thecollection region for electrons and holes to recombine to generatelight. The Ge 3201 & 3202 also serve as the optical confinement layerand the waveguide material. The GaAs 3204 & 3205 serves as theemitter/cladding and collector/cladding material for this structure. Thecladding serves as funneling carriers into the active/waveguide regionand traps the emit light in the waveguide structure. The large energybandgap difference between the GaAs 3204 & 3205 and the Ge 3201 & 3202ensures a large index of refraction difference at the N⁻emitter/cladding 3206 and P⁺ base/barrier 3207 junction and a largeindex refraction difference at the P⁺ base/barrier 3208 and N⁻collector/cladding 3209, thus making an excellent waveguide 3210 tooptically confine the light produced by the active region.

FIG. 33 shows a possible cross-sectional device depiction of an NPNtransistor laser structure with a GeSn QW or QD 3306 active region in aGe P-type base/barrier material 3300. The structure can be grown on N⁺GaAs conducting substrate 3303, which is the seed crystal to grow thefull structure. N⁻ GaAs collector/cladding 3304 and the N⁻ GaAsemitter/cladding 3308 do dual functions of optical confinement of thelight 3309 produced and controlling the flow of electrons and holes. TheP⁺ Ge Base 3305 and 3307 form the barrier material for the GeSn QW or QD3306, and also provide the waveguide material. The laser can require aresonant cavity to get optical gain, and typically this can be formedfrom the front cleaved facets 3302 and back cleaved facets 3301 of thesemiconductor crystalline structure.

Table 2 shows an exemplary epitaxial structure of an NPN light emittingwith a GeSn QW or QD active region in a Ge P-type base/barrier HBT.

TABLE 2 Exemplary epitaxial structure of NPN light emitting with a GeSnQW or QD active region in a Ge P-type base/barrier HBT. Layer Layer NameDescription Comment 1 N⁺ Cap ~1000 Å InGaAs (Te-doped >10¹⁹ cm⁻³) 2 N⁻Emitter/Cladding ~1000 Å GaAs (Si-doped~3 × 10¹⁷ cm⁻³) 3 P⁺ Base/Barrier~450 Å Ge (B-doped >10¹⁹ cm⁻³) (B = boron) 4 QW or QD ~100 Å GeSn Forlight emission Sn content can be: 0 ≤ Sn % ≤ 20% 1000 nm-5000 nm QW (Sn%~0% to 20% GeSn) QW thickness range 10 Å-1000 Å QD (Sn %~0% to 20%GeSn) QD size range 10 Å-500 Å 5 P⁺ Base/Barrier ~450 Å Ge (B-doped>10¹⁹ cm⁻³) 6 N⁻ Collector ~1000 Å GaAs (Si-doped~1 × 10¹⁶ cm⁻³) 7 N⁺Sub-Collector ~500 Å GaAs (Si-doped~5 × 10¹⁸ cm⁻³) 8 N⁺ Buffer ~500 ÅGaAs (Si-doped~5 × 10¹⁸ cm⁻³) 9 GaAs N⁺ conducting substrate Crystalline

The semiconductor alloy In_(0.49)Ga_(0.51)P (InGaP) can be latticematched to GaAs. InGaP can be grown in a disordered phase, orderedphase, or a combination of the two. The disordered InGaP phase has abandgap energy of 1.9 eV. The bandgap of the ordered InGaP is about 1.85eV. In some examples, to fabricate a light emitting bipolar transistormay require an insertion into the GaAs base region a GeSn quantum well,quantum dot, or quantum wire layer. The GaAs is the p type base materialbut also acts as a barrier layer to quantum confine the electrons andholes in the GeSn QW. QWs are formed by having a large energy bandgapmaterial surrounded by a low energy bandgap material which results intwo dimensional electron confinement. For a QD the strained layer growthresults in three dimensional electron confinement. Typical thicknessesof quantum wells are about 100 Å but they could be larger or less thanthat thickness depending on the emission wavelength desired. Typicalquantum dot diameters are in the range of 1 nm-20 nm, but are dependenton the wavelength of light that needs to be emitted.

FIG. 34 shows an exemplary flat band energy diagram of an NPN transistorlaser structure with a GeSn QW or QD active region in a GaAs P⁺base/barrier material 3400. The In_(0.49)Ga_(0.51)P disordered 3401 &3405 serves as the N⁻ emitter/cladding and N⁻ collector/cladding layers.Note the relevant band discontinuities ΔE_(C), ΔE_(V) are shown in thefigure. The laser includes a GeSn QW or QD 3403 active region for thecollection region for electrons and holes to recombine to generatelight, which is inserted into a P⁺ GaAs base/barrier 3402 & 3404, whichalso serves as the barrier layer of the GeSn QW or QD 3403 activeregion, thus serving also as the optical waveguide 3410 material. Thelarge energy bandgap difference between the In_(0.49)Ga_(0.51)Pdisordered 3401 & 3405 and the GaAs base/barrier 3402 & 3404 ensures alarge index of refraction difference at the N emitter/cladding 3406 andP⁺ base/barrier 3407 junction and a large index refraction difference atthe P⁺ base/barrier 3408 and N⁻ collector/cladding 3409, thus making anexcellent waveguide 3410 to optically confine the light produced by theactive region.

FIG. 35 shows a possible cross-sectional device depiction of an NPN edgeemitting transistor laser structure with a GeSn QW or QD active regionin a GaAs P⁺ base/barrier material 3500. A GeSn QW or QD 3506 activeregion in a P⁺ GaAs base 3505 & 3507 which also functions as the barriermaterial for the QW or QD. This is an exceptional device because thetype I discontinuities form an excellent optical and electricalconfining structure. The structure can be grown on N⁺ GaAs conductingsubstrate 3503, which is the seed crystal to grow the full structure. N⁻InGaP (disordered) collector/cladding 3504 and the N⁻ InGaP (disordered)emitter/cladding 3508 do dual functions of optical confinement of thelight 3509 produced and controlling the flow of electrons and holes. P⁺GaAs base 3505 & 3507 form the barrier material for GeSn QW or QD 3506,and also provide the waveguide material. The laser can require aresonant cavity to get optical gain, and typically this can be formedfrom the front cleaved facets 3502 and back cleaved facets 3501 of thesemiconductor crystalline structure.

Table 3 shows an exemplary table of the epitaxial structure of an NPNlight emitting GeSn QW or QD active region in a GaAs P-type base/barrierHBT.

TABLE 3 An Exemplary epitaxial structure of an NPN edge emittingtransistor laser structure with a GeSn QW or QD active region in a GaAsP-type base/barrier material. Layer Layer Name Description Comment 1 N⁺Cap ~1000 Å InGaAs (Te-doped >10¹⁹ cm⁻³) 2 N⁻ Emitter/Cladding ~5000 ÅIn_(0.49)Ga_(0.51)P (Si-doped~3 × 10¹⁷ cm⁻³) Disordered 3 P⁺Base/Barrier ~500 Å GaAs (B-doped >10¹⁹ cm⁻³) 4 QW or QD ~55 Å GeSn Forlight emission Sn content can be: 0 ≤ Sn % ≤ 20% 1000 nm-5000 nm QW (Sn%~0% to 20% GeSn) QW thickness range 10 Å-1000 Å QD (Sn %~0% to 20%GeSn) QD size range 10 Å-500 Å 5 P⁺ Base/Barrier ~500 Å GaAs (B-doped>10¹⁹ cm⁻³) 6 N⁻ Collector/Cladding ~5000 Å In_(0.49)Ga_(0.51)P(Si-doped~3 × 10¹⁷ cm⁻³) Disordered 7 N⁺ Sub-Collector ~500 Å GaAs(Si-doped~5 × 10¹⁸ cm⁻³) 8 N⁺ Buffer ~500 Å GaAs (Si doped~5 × 10¹⁸cm⁻³) 9 N⁺ GaAs conducting substrate Crystalline

The transistor laser has the integrated features of both the transistorand the laser. From such a structure it can be straightforward tosimplify the structure and grow a separate confinement heterostructure(SCH) laser.

FIG. 36 shows an exemplary flat band energy diagram of an SCH laserutilizing a GeSn QW or QD region located in UID Ge barrier/OCL layer3602 & 3604 region with P⁺ GaAs 3601 cladding and N⁺ GaAs 3605 claddinglayers. This structure represents a PN junction or diode with anunintentionally doped (UID) active region and optical confinement regionbetween the P⁺ GaAs 3601 and N⁺ GaAs 3605 cladding. The UID Ge 3602 and3604 forms the barrier material for the GeSn QW or QD 3603 activeregion. The combination of the barrier and active region forms thewaveguide 3606 of the laser. The P⁺ GaAs 3601 cladding region serves forinjection of the holes and for the optical confinement of the lightemitted from the active region. The N⁺ GaAs 3605 cladding region servesfor injection of the electrons and for the optical confinement of thelight emitted from the active region. Though this depicts a symmetricstructure it can be also asymmetric.

FIG. 37 shows a cross-sectional depiction of a SCH ridge laser utilizinga GeSn QW or QD region located in UID Ge barrier/OCL region with P-typeGaAs and N-type GaAs cladding. The structure can be grown on N⁺ GaAsconducting substrate 3703. N⁺ GaAs cladding 3704 serves for theinjection of electrons into the active region and the bottom claddingfor the optical confinement of the light emitted from the active region.The P⁺ GaAs cladding 3708 serves for the injection of holes into theactive region, and additionally, the top cladding for opticalconfinement of the light 3709 emitted from the active region. The UID Ge3705 & 3707 forms the barrier material for the GeSn QW or QD 3706, andalso provide the OCL material. The laser can require a resonant cavityto get optical gain, and typically this can be formed from the frontcleaved facets 3702 and back cleaved facets 3701 of the semiconductorcrystalline structure. The ridge structure provides the vertical guidingof the current into the active region.

Table 4 shows an exemplary epitaxial structure SCH injection diode laserwith a GeSn QW or QD region with Ge barriers.

TABLE 4 Exemplary epitaxial structure SCH injection diode laser with aGeSn QW or QD region with Ge barriers. Layer Layer Name DescriptionComment 1 P⁺ Cap ~1000 Å GaAs (Zn doped >10¹⁹ cm⁻³) 2 P⁺ Cladding ~10000Å GaAs (Zn doped ~1 × 10¹⁸ cm⁻³) 3 Barrier/OCL ~450 Å Ge UID 4 QW or QD~100 Å GeSn For light emission Sn content can be: 0 ≤ Sn % ≤ 20% 1000nm-5000 nm QW (Sn %~0% to 20% GeSn) QW thickness range 10 Å-1000 Å QD(Sn %~0% to 20% GeSn) QD size range 10 Å-500 Å 5 Barrier/OCL ~450 Å GeUID 6 N⁺ Cladding ~10000 Å GaAs (Si-doped~1 × 10¹⁸ cm⁻³) 7 N⁺ Buffer~5000 Å GaAs (Si-doped~5 × 10¹⁸ cm⁻³) 8 GaAs N⁺ conducting substrateCrystalline

A variation of the laser structure could incorporate GeSn QW or QDregion in a UID GaAs barrier/waveguide region, and utilizing latticematched InGaP as the cladding material. FIG. 38 shows an exemplary flatband energy diagram of an SCH diode laser utilizing a GeSn QW or QDregion located in UID GaAs 3802 & 3804 barrier/OCL region with P⁺ InGaP3801 disordered and N⁺ InGaP 3805 disordered cladding. This structurerepresents a PN junction or diode with an unintentionally doped (UID)active region and optical confinement region between the P⁺ InGaP 3801disordered and N⁺ InGaP 3805 disordered cladding. The UID GaAs 3802 &3804 forms the barrier material for the GeSn QW or QD 3803 activeregion. The combination of the barrier and active region forms thewaveguide 3806 of the laser. The P⁺ In_(0.49)Ga_(0.51)P disordered 3801top cladding region serves for injection of the holes and for theoptical confinement of the light emitted from the active region. The N⁺In_(0.49)Ga_(0.51)P disordered 3805 bottom cladding region serves forinjection of the electrons and for the optical confinement of the lightemitted from the active region. Though this depicts a symmetricstructure it can be also asymmetric.

Table 5 shows an exemplary epitaxial structure SCH injection diode laserwith a GeSn QW or QD region with GaAs barriers.

TABLE 5 Exemplary epitaxial structure SCH injection diode laser with aGeSn QW or QD region with GaAs barriers. Layer Layer Name DescriptionComment 1 P⁺ Cap ~1000 Å GaAs (Zn doped >10¹⁹ cm⁻³) 2 P⁺ Cladding ~10000Å In_(0.49)Ga_(0.51)P (Zn-doped~1 × 10¹⁸ cm⁻³) Disordered 3 Barrier/OCL~450 Å GaAs UID 4 QW or QD ~10-1000 Å GeSn For light emission Sn contentcan be: 0 ≤ Sn % ≤ 20% 1000 nm-5000 nm QW (Sn %~0% to 20% GeSn) QWthickness range 10 Å-1000 Å QD (Sn %~0% to 20% GeSn) QD size range 10Å-500 Å 5 Barrier/OCL ~450 Å GaAs UID 6 N⁺ Cladding ~10000 ÅIn_(0.49)Ga_(0.51)P (Si-doped~1 × 10¹⁸ cm⁻³) Disordered 7 N⁺ Buffer~5000 Å GaAs (Si-doped~5 × 10¹⁸ cm⁻³) 8 GaAs N⁺ conducting substrateCrystalline

The lattice constant of GaAs and Ge is both about 5.65 Å. The GeSnlattice constant can change from 5.66 Å to 5.833 Å at about 20% Sncontent, the range of lattice mismatch at the highest Sn content isabout 3%. This makes GeSn useful for growth on Ge or GaAssemiconductors, because at low Sn % GeSn can be grown coherentlystrained or pseudomorphic on either Ge or GaAs. For low Sn % GeSn thelattice mismatch is reasonable and films can be grown pseudomorphic(strained) if thin enough, or partial relaxation may occur for thickerfilms (1000 Å or more). Thus for growth of GeSn QW on Ge or GaAs, planargrowth can be achieved. FIG. 39 shows the planar growth of strained GeSn3901 (low Sn %) on Ge, with Ge barriers above and below the QW GeSnfilm. For low Sn % GeSn either Ge or GaAs barriers can be used for theformation of the strained GeSn QW.

The formation of quantum dot structures are a result of the ability ofself-assembled GeSn quantum dots by the Stranski-Krastanov (SK) methodthat transitions from two dimensional to island growth. FIG. 40 showsthe methodology of formation. A larger lattice constant semiconductor isgrown on a semiconductor with a smaller lattice constant. Under properconditions, two dimensional planar growth starts but quickly transitionsto island growth, thus forming the quantum dot structure. Thus thelattice constant of GeSn is typically greater than that of Ge. Typicallythe formation of the quantum dots occurs when the critical thickness ofthe GeSn layer is exceeded. The lattice mismatch should be typicallygreater than 2% for dot formation. The range of lattice mismatch of GeSnto that of Ge (or GaAs) approximately starts at 0% and goes up to 3%lattice mismatch at 20% Sn in GeSn. FIG. 40 shows the island growth ofstrained GeSn 4001 (high Sn %) on Ge barrier layer with the subsequentformation of the QD layer.

GeSn quantum structure provide a unique methodology to form both QW andQD in the same structure because the Sn % for becoming a direct gapsemiconductor can vary from 7%≤Sn %≤20%. GeSn is indirect up to about 7%Sn then becomes a direct gap semiconductor with an energy bandgap of0.585 eV and a lattice constant of about 5.725 Å, and the bandgap energytypically reduces to about 0.25 eV at 20% Sn with a lattice constant ofabout 5.835 Å. Utilizing Ge or GaAs barriers which have a latticeconstant of about 5.65-5.66, one can calculate the lattice mismatch atvarious compositions. The lattice mismatch between GeSn to GaAs or Ge at7% Sn content GeSn is about 1%. The lattice mismatch between GeSn toGaAs or Ge at 20% Sn content in GeSn is about 3%. Typically theformation of the quantum dots generally occurs when the criticalthickness of the GeSn layer is exceeded. The lattice mismatch should betypically greater than 2% for quantum dot formation. A 2% latticemismatch of Ge or GaAs to the GeSn corresponds to a lattice constant ofabout 5.76 Å which is about 12% Sn in GeSn. Thus if one grows on Ge orGaAs barriers, one can get GeSn planar direct gap Type I QW for 7%≤Sn%≤12% and direct gap type I QD for 12%≤Sn %≤20%. The GeSn energies varyfrom 7% Sn with a bandgap energy of 0.585 eV; to 12% Sn with a bandgapenergy of 0.48 eV; to 20% Sn with a bandgap energy of about 0.25 eV.Thus GeSn QW energies could be in the near-IR and the GeSn QD energiescould be in the mid-IR, utilizing the exact same laser or transistorlaser structure.

Exemplary Configuration 2A: Ordered InGaP Emitter—GeSn Base—GaAsCollector double heterojunction transistor. The device elucidated caninclude a double heterojunction InGaP—GeSn—GaAs HBT device. InGaP at thecomposition In_(0.49)Ga_(0.51)P is lattice matched to GaAs and is adirect gap semiconductor. In_(0.49)Ga_(0.51)P can be grown in two formsordered and disordered. InGaP semiconductor grown by various epitaxialgrowth technologies can be lattice matched to GaAs. At high temperaturegrowth the InGaP can grow in a crystalline structure such that thesheets of In—P and Ga—P atoms can alternate in the (001) planes of theface centered cubic (FCC) unit cell without the intermixing of the Gaand In atoms on the lattice planes. Such an arrangement results in analmost zero conduction band discontinuity between the InGaP and GaAs andis called the ordered phase (this can be of weakly type I or weakly typeII because it is close to zero). The ordered phase has a bandgap energyof 1.85 eV and the disordered phase has a bandgap energy of 1.9 eV, thusthe ordered phase has a bandgap energy of about 0.05 eV less than thedisordered phase. With different growth conditions, the In and Ga atomscan intermix and the disordered InGaP phase can form, which has a largerconduction band offset of 0.1 eV (type I) vs. 0.03 eV for the orderedphase. It is also possible to have a mixture of ordered and disorderedInGaP materials. The lattice constant of GaAs and Ge is about 5.66 Å,and this is also the lattice constant of InGaP at the compositionIn_(0.49)Ga_(0.51)P.

In some examples, the ordered phase may have an advantage to thedisordered phase, because the ordered phase has near zero conductionband offset. In some examples, this device has desirable basecharacteristics with a low voltage base turn-on region and that the GeSnbase region can be directly inserted into a standard InGaP—GaAs HBTs,which is typically used in RF power amplifiers in wireless devices andcellular handsets to send the voice and data to the cell tower.Additionally, in an inverted HBT structure by using the ternary alloyInGaP as the emitter and varying the In composition away from thelattice matched condition, strain can be introduced into the GeSn baselayer, thus the GeSn layer can be tensile or compressively strained.

FIG. 41 shows an exemplary flat band energy diagram of an NPN HBT withan ordered InGaP emitter, GeSn base, and a GaAs collector. This type ofHBT structure is called an asymmetric double heterojunction device. Noteconduction band offsets are less than 0.1 eV and are in the near zeroconduction band offset range and are desirable for NPN transistors. Theordered In_(0.49)Ga_(0.51)P 4101 (disordered InGaP can also be usedhere) is an excellent material for the emitter because it has a smallconduction band offset and a large valence band offset with GeSn 4102 P⁺base region. The N-type GaAs 4103 is a proven collector material forHBTs.

Additionally, the base can be graded from Ge—GeSn 4201 to have electricfield enhancement of the charge carriers (electrons). Such structurecreates an electric field that accelerates the electrons across the baseto the collector. FIG. 42 shows an exemplary flat band energy diagram ofan NPN HBT with an ordered InGaP N emitter, a graded Ge—GeSn 4201 P⁺base, and a GaAs N collector. The graded Ge—GeSn 4201 layer can compriseat the emitter a Ge or a low Sn % GeSn layer which is graded to higherSn % GeSn at the collector interface. The grading range can go from Geat the emitter to GeSn at various compositions up to 20%. The grading ofthe base provide for a field enhancement 4202 region. One could grow theemitter first and then grade the Ge to GeSn.

Table 6 shows an exemplary epitaxial structure of an NPN HBT with anordered InGaP emitter, GeSn base region, and a GaAs collector.

TABLE 6 Epitaxial structure of NPN HBT with ordered InGaP emitter, GeSnbase, and GaAs collector. Layer Layer Name Description Comment 1 N⁺ Cap~1000 Å InGaAs (Te-doped >10¹⁹ cm⁻³) 2 N⁻ Emitter Cap ~1500 Å GaAs(Si-doped~5 × 10¹⁸ cm⁻³) 3 N⁻ Emitter ~500 Å In_(0.49)Ga_(0.51)P(Si-doped~3 × 10¹⁷ cm⁻³) Ordered 4 P⁺ Base ~500 Å GeSn (B-doped >10¹⁹cm⁻³) Or graded 0 ≤ Sn % ≤ 20% Ge-GeSn Thickness range 100 Å-5000 Å 5 N⁻Collector ~10000 Å GaAs (Si-doped~1 × 10¹⁶ cm⁻³) 6 N⁺ Sub-Collector~5000 Å GaAs (Si-doped~5 × 10¹⁸ cm³) 7 High Purity Buffer ~500 Å GaAs(un-doped) UID 8 GaAs semi-insulating substrate Crystalline

It should be noted that there are many types of N-type and P-typedopants. For standard III-V semiconductors like GaAs, InP, InGaAs,InGaP, the N-type dopants are Si, Ge, Sn, Pb, S, Se, Te. The P-typedopants for standard III-V semiconductors are C, Zn, Be, Mg. Commondopants for group IV semiconductors like GeSn, Ge, Si, SiGe, GeSiSn forN⁻ type dopants are P, As, Sb. The P-type dopants are B, Al, Ga.

Ge used as a base material in HBTs has many advantages. Ge has thefollowing properties that make it an excellent P-type base. Ge has a lowbandgap (the term low energy bandgap base typically refers to therelevant semiconductors with bandgaps less than 0.75 eV, like GeSn, Ge,InGaAs, and GaAsSb) which results in a low turn-on voltage. The Ge holemobility is high (2000 cm²/Vs) and acceptors can be incorporated to highdensity (>1×10¹⁹ cm⁻³), thus the base can be made ultra-thin (less than5000 Å) while maintaining a low base sheet resistance (<<18 Ohm/sq)which increases current gain and decreases electron transit time. Ge hastrue shallow acceptors, so that the hole concentration is generallyequal to the acceptor doping level and independent of temperature. Gehas a low base resistivity (0.0002 ohm-cm) which results in a highF_(max). The surface recombination velocity is low for P-type Ge. Lowresistance ohmic contacts can be formed on P-type Ge. Ge mobility(electron and hole) can be significantly improved by being biaxiallytensile strain, thus for both NPN and PNP structures the base sheetresistance can be improved significantly. The Ge hole mobility undertension can exceed 10,000 cm²/Vs and under biaxial compression of Geenhances the hole mobility but degrades the electron mobility. Thus Geis a desirable base material and thus GeSn will have similar properties.

Typically for HBTs the collector is grown first followed by growing thebase and then growing the emitter. However, for this structure, it canbe advantageous to grow an inverted HBT. By growing theIn_(0.49)Ga_(0.51)P emitter first, one could increase the indium contentto greater than 49% In, thus, the bandgap energy would be reduced, butthe lattice constant would be increased. The conduction band offsetbetween the InGaP (In %>49) would cause the bands to be closer to zerooffset between the InGaP and Ge. This methodology could be applied to aGe base. FIG. 43 shows an exemplary flat band energy diagram of aninverted NPN HBT structure where the emitter is grown first, and thebase material is tensile strained Ge. This Inverted HBT structureemitter grown first collector up 4300 has a field enhancement region4303 in the compositionally graded InGaP 4301 N⁻ emitter. This structureallows for the P⁺ base to be tensile strained Ge 4302, thus enhancingthe hole mobility. Additionally, one could decrease the In % in theInGaP and then the Ge would be biaxially compressively strained.

The device structure advantages: By growing the emitter InGaP on GaAs,one can initially lattice matched the InGaP to the GaAs. When the Incomposition can be increased to the point where Ge is biaxially tensilestrained to about 1.75% it then becomes a direct gap semiconductor.

Tensile Strain effects on Ge: It has shown that biaxial tensilecompression causes enhancements in the hole and electron mobility.Biaxial tension on the band structure of Ge breaks the heavy hole andlight hole band degeneracy and raises the light hole above the heavyhole band. This effectively increases the hole mobility. Typically theGe band structure shows that it is an indirect semiconductor because the“L” point <111> is the conduction band minimum and the gamma “Γ” pointis the valence band maximum. However Ge becomes direct gap semiconductorwith 1.4% biaxial tensile strain or greater, because the gamma “Γ” pointin Ge band structure gets closer to the valence band maximum faster thanthe “L” point <111>, thus making it a direct band gap semiconductor.

With biaxial tensile strain a dramatic increase in the Ge hole mobility“μ_(h)”. Thus, by growing an inverted emitter structure one caneffectively tensile or compressive strain the Ge (or GeSn) layer with nodegradation in performance. It has been shown experimentally thatbiaxial tensile can increase the in-plane hole mobility at 3% biaxialstrain of a Ge hole mobility>40,000 cm²/Vs. If the InGaP is graded tohigher In % then an electric field can be built-in that can promote freecharge carriers from the emitter into the base region.

Compressive strain effects on Ge: Ge under biaxial compression shows anenhancement in the in-plane hole mobility. It has been shownexperimentally that biaxial compressive strain of 1.7% in the Ge layerincreases the low field hole mobility by a factor of 3.38 to 6350cm²/V−s.

The elucidated tensile and compressive strain effects should also workfor GeSn for low compositions of Sn %. FIG. 44 shows an exemplary flatband energy diagram showing an inverted tensile strained GeSn HBTstructure emitter grown first collector up 4400. This NPN HBT structurewhere the emitter is grown first, and the base material is tensilestrained GeSn 4402 has some advantages. This inverted HBT structure hasa field enhancement region 4303 in the graded InGaP 4301 N⁻ emitter.This structure allows for the P⁺ base to be tensile strained GeSn 4402,thus enhancing the hole mobility. By utilizing InGaP in thisconfiguration one can strain the GeSn in tension or compression.

If an inverted structure is not desirable, the collector grown firststructure (emitter up) can be grown and the InGaP collector can begraded as follows: from the GaAs sub-collector the InGaP starts at 49%In, then is slowly graded up to an In % greater than 49% at the start ofthe GeSn base region. This will result in field enhancement region inthe collector to accelerate the electrons to the sub-collector. FIG. 45shows an exemplary flat band energy diagram of an NPN configuration,compressively strained GeSn HBT collector grown first emitter up 4500structure, where the compressive strained GeSn 4501 is grown on the N⁻collector InGaP 4502 compositionally graded from In 49% to >49%collector which now has a field enhancement region 4503. This device hasthe advantage that it is a true double heterojunction almost symmetricdevice. This would minimize the offset voltage found in standardInGaP—GaAs HBTs that causes a reduction in power added efficiency. Notethat compressively strained Ge could also be used as the base region inthis device.

An additional variation of this device results in GeSn that may or maynot be biaxially strained, by having both the emitter and collectorInGaP layers compositionally graded. Basically this is a combination ofthe previously described embodiments for the strained GeSn HBTs. Herethe emitter and collector both have field enhancement regions becausethe InGaP is graded in both layers. The standard configuration where thecollector is grown first (emitter up) and the InGaP collector can begraded as follows: from the GaAs sub-collector the InGaP starts at 49%In, then is slowly graded up to an In % greater than 49% at the start ofthe GeSn base. This process is repeated for the emitter where InGaPstarts at 49% In, then is slowly graded up to an In % greater than 49%at the start of N emitter contact region. FIG. 46 shows an exemplaryflat band energy diagram of a GeSn Double HBT Structure graded Emitterand graded Collector grown first 4600, where the GeSn 4605 base may ormay not be compressively strained. The N⁻ emitter InGaP 4601 is gradedfrom In % 49% to >49% has a field enhancement region 4602. The N⁻collector InGaP 4603 is graded from In % 49% to <49% has a fieldenhancement region 4604. The field enhancement regions cause anacceleration of the electrons to the NPN device, thus reducing thetransit time of the device.

Note through this patent InGaP is used throughout the text. Whereordered InGaP is referred to an equivalent device using disordered InGaPcan be used. Likewise where disordered InGaP is used ordered InGaP canalso be used. Though at some instance the lattice matched composition toGaAs is used In_(0.49)Ga_(0.51)P. Though this is a useful “In”composition for starting the InGaP layer, it can be graded or have adifferent composition.

Exemplary Configuration 2B: NPN Disordered InGaP Emitter—GeSn Base—GaAsCollector double heterojunction transistor: The device elucidated caninclude a double heterojunction disordered In_(0.49)Ga_(0.51)P—GeSn—GaAsHBT device. In some examples, this device has desirable basecharacteristics with a low voltage base turn-on region. This devicestructure can be directly inserted into standard manufacturingInGaP—GaAs HBT. This is a slight variation on Configuration 2A. Thedifference is the conduction band offset of disordered InGaP to GeSn isabout 0.15 eV (ordered InGaP was about 0.08 at its low Sn %), and thevalence band offset of the disordered InGaP to GeSn (low Sn %) is 1.09eV (ordered InGaP was about 1.11 eV at low Sn %). In some instances itcan be easier to grow the disordered InGaP.

Exemplary Configuration 2C: NPN AlGaAs Emitter—GeSn Base—GaAs Collectordouble heterojunction transistor: The device elucidated can include adouble heterojunction AlGaAs—GeSn—GaAs HBT device. In some examples,this device has desirable base characteristics with a low voltage baseturn-on region. This is a second variation on Configuration 2A. ForAlGaAs for Al % less than 0.4 the material is direct gap semiconductor.The energy band gap of Al_(0.3)Ga_(0.7)As, a typical emittercomposition, is 1.8 eV (direct gap) as opposed In_(0.49)Ga_(0.51)P,which has an energy band gap of 1.85 eV (direct gap). The difference isthe conduction band offset of Al_(0.3)Ga_(0.7)As to GeSn (low Sn %) is0.29 eV (ordered InGaP was 0.08), and the valence band offset ofAl_(0.3)Ga_(0.7)As to GeSn (at low Sn %) is about 0.85 eV (ordered InGaPwas 1.11 eV). FIG. 47 shows an exemplary flat band energy diagram of theNPN AlGaAs Emitter—GeSn Base—GaAs Collector double HBT 4700. In this HBTthe N emitter is Al_(0.3)Ga_(0.7)As 4701 but the Al % could be varied todifferent levels for optimized transport.

To fabricate a light emitting NPN heterojunction bipolar transistor inthis Al_(0.3)Ga_(0.7)As (InGaP could also be used as the emitter andcollector) emitter HBT, a GeSn QW or QD region can be inserted into theGaAs base (Ge could also be used as the base). FIG. 48 shows anexemplary flat band energy diagram of the NPN HBT laser with AlGaAsemitter/cladding and AlGaAs collector/cladding 4800. This is anexceptional device because the type I alignment allows for excellentcarrier and optical confinement. The device structure is symmetric. TheN⁻ Al_(0.3)Ga_(0.7)As 4801 & 4805 is an excellent cladding layer and hassmall mismatch with GaAs. The P⁺ GaAs base 4802 & 4804 acts as thebarrier layer for the GeSn QW or QD 4803 active region. The large energybandgap difference between the N⁻ Al_(0.3)Ga_(0.7)As 4801 & 4805 and theP⁺ GaAs base 4802 & 4804 ensures a large index of refraction differenceat the N⁻ emitter/cladding 4807 and P⁺ GaAs 4802 base interface; and alarge index refraction difference at the P⁺ GaAs base 4804 and N⁻collector/cladding 4808, thus making an excellent waveguide 4806 tooptically confine the light produced by the active region. The laser canrequire a resonant cavity to get optical gain, and typically this canformed from the front and back cleaved facets of the semiconductorcrystal wafer.

Table 7 shows an exemplary table of the epitaxial structure of an NPNlight emitting AlGaAs—GaAs—GeSn—GaAs—AlGaAs HBT.

TABLE 7 An exemplary epitaxial structure of an NPN edge emittingtransistor laser structure with a GeSn QW or QD active region in a GaAsP-type base/barrier material and AlGaAs Cladding. Layer Layer NameDescription Comment 1 N⁺ Cap ~1000 Å InGaAs (Te-doped >10¹⁹ cm⁻³) 2 N⁻Emitter/Cladding ~5000 Å Al_(0.3)Ga_(0.7)As (Si-doped~3 × 10¹⁷ cm⁻³)Different Al% AlGaAs can be used 3 P⁺ Base/Barrier ~500 Å GaAs (B-doped>10¹⁹ cm⁻³) 4 QW or QD ~55 Å GeSn For light emission Sn content can be:0 ≤ Sn % ≤ 20% 1000 nm-5000 nm QW (Sn %~0% to 20% GeSn) QW thicknessrange 10 Å-1000 Å QD (Sn %~0% to 20% GeSn) QD size range 10 Å-200 Å 5 P⁺Base/Barrier ~500 Å GaAs (B-doped >10¹⁹ cm⁻³) 6 N⁻ Collector/Cladding~5000 Å Al_(0.3)Ga_(0.7)As (Si-doped~3 × 10¹⁷ cm⁻³) Different Al %AlGaAs can be used 7 N⁺ Sub-Collector ~500 Å GaAs (Si-doped~5 × 10¹⁸cm⁻³) 8 N⁺ Buffer ~500 Å GaAs (Si-doped~5 × 10¹⁸ cm⁻³) 9 N⁺ GaAsconducting substrate Crystalline

Exemplary Configuration 3: An NPN and PNP GeSiSn Emitter—Ge Base—GeSiSnCollector symmetric double heterojunction transistor. This deviceconfiguration is different because GeSiSn can be lattice matched to Ge,even though the GeSiSn has a larger bandgap energy than Ge. In addition,because the ternary alloy GeSiSn can be grown at various compositions,it is possible to also biaxial tensile strain or compressive strain theGe base region (lattice constants above and below Ge). For GeSiSn the Sn% and Si % can be adjusted so that the lattice parameter remainsconstant. Also P-type and N-type doping have been achieved in GeSiSn.GeSiSn can be grown on Si, GaAs, Ge substrates. For exemplaryConfiguration 3, Si substrates will be a possible choice. Also a GeSnlayer can be used as the base material.

For Si based HBTs, GeSiSn is a unique semiconductor alloy because it canbe lattice matched to Ge at the compositionGe_(1-x)(Si_(0.8)Sn_(0.2))_(x), where “x” can vary from 0 to 0.5 and thedirect gap energy of this material can vary from 0.8 eV to 1.24 eV. ThusGeSiSn is an excellent emitter for a Si HBT or a barrier layer for a Gequantum well or quantum dot (additionally for GeSn quantum well orquantum dot), because it can be lattice matched to Ge or cancompressively strain the Ge thus promoting island growth necessary forquantum dot formation. By lowering the Si to Sn ratio in GeSiSn thelattice constant can be decreased. The GeSiSn can also be latticematched to GeSn or can tensile strain or compressively strain the GeSnlayer.

FIG. 49 shows a possible exemplary flat band energy band diagram for asymmetric double HBT GeSiSn emitter—Ge base—GeSiSn collector structure4900 which can work as an NPN or PNP transistor device. Note that a GeSnlayer can be used as the base with similar results. However, becauseGe_(1-x)(Si_(0.8)Sn_(0.2))_(x) 4901 & 4903 can be grown at differentalloy (x) compositions, both compressive and tensile strain can beapplied to the Ge 4902 base, both configurations PNP and NPN are useful.This figure shows the flat “Γ” band edge energy diagram of the materialstructure.

Additionally, the base can be graded from Ge to GeSn to have electricfield enhancement of the charge carriers (electrons and holes) as shownin the “Γ” band edge diagram of FIG. 50. FIG. 50 shows a possibleexemplary flat band energy band diagram for GeSiSn emitter—gradedGe—GeSn base—GeSiSn collector structure double HBT 5000 which can workas an NPN or PNP transistor device. With the graded Ge—GeSn 5001 baseregion a field enhancement region 5002 is created in the base. Suchstructure creates an electric field that accelerates the electrons andholes across the base to the collector. The graded Ge—GeSn 5001 layermay comprise at the emitter a Ge or a low Sn % GeSn layer which isgraded to higher Sn % GeSn at the collector interface. The grading rangecan go from Ge at the emitter to GeSn at various compositions up to 20%.

Table 8 shows a possible exemplary structure for a symmetric doubleheterojunction GeSiSn emitter—Ge base—GeSiSn collector structure whichcan work as an NPN device. If the GeSiSn lattice constant is made largerthan the Ge lattice constant, the Ge can be tensile strained (similar tothe InGaP emitter and collector situation). This causes the light holeband to rise above the heavy hold band in the valence band and resultsin a significant enhancement in the P-type Ge base mobility and, thus,the same base thickness the base sheet resistance can be reduced and thehigh frequency performance of the transistor is (F_(max)) increased.Additionally, because the hole mobility is enhanced, the baseresistivity will be reduced, thus one has the additional option toreduce the thickness of the base while keeping the base sheet resistanceunchanged. A thinner base promotes F_(T) to increase. In this exemplarystructure the base could be a P⁺ GeSn layer.

TABLE 8 Epitaxial structure of an NPN heterojunction GeSiSn emitter-Gebase-GeSiSn collector. Layer Layer Name Description Comment 1 N⁺ EmitterCap ~1500 Å SiGe (As-doped~5 × 10¹⁸ cm⁻³) 2 N⁻ Emitter ~500 ÅGe_(1-x)(Si_(0.8)Sn_(0.2))_(x) (As-doped~3 × 10¹⁷ cm⁻³) 3 P⁺ Base ~500 ÅGe (B-doped >10¹⁹ cm⁻³) Or can be Or GeSn, Sn content 0 ≤ Sn % ≤ 20%graded Thickness range 100 Å-5000 Å Ge-GeSn for field enhancement 4 N⁻Collector ~10000 Å Ge_(1-x)(Si_(0.8)Sn_(0.2))_(x) (As-doped~1 × 10¹⁶cm⁻³) 5 N⁺ Sub-Collector ~5000 Å SiGe (As-doped~5 × 10¹⁸ cm⁻³) SiGe Gecontent can be varied to accomodate the Ge_(1-x)(Si_(0.8)Sn_(0.2))_(x)layer 6 N⁺ Buffer ~500 Å Si (As-doped~2 × 10¹⁸ cm⁻³) 7 N⁺ Si substrateCrystalline

Table 9 shows a possible exemplary structure for a symmetric doubleheterojunction GeSiSn emitter—Ge base—GeSiSn collector structure whichcan work as a PNP device. If the GeSiSn lattice constant is made largerthan the Ge lattice constant then the Ge can be tensile strained. Thiscauses the light hole band in the valence to split from the heavy holeband and results in an enhancement in the P-type Ge base mobility, thusreducing the base sheet resistance and increasing the high frequencyperformance of the transistor. In this exemplary structure the basecould be a heavily N⁺ GeSn layer.

TABLE 9 Epitaxial structure of a PNP heterojunction GeSiSn emitter-Gebase-GeSiSn collector. Layer Layer Name Description Comment 1 P⁺ EmitterCap ~1500 Å SiGe (B-doped~5 × 10¹⁸ cm⁻³) 2 P⁻ Emitter ~500 ÅGe_(1-x)(Si_(0.8)Sn_(0.2))_(x) (B-doped~3 × 10¹⁷ cm⁻³) 3 N⁺ Base ~500 ÅGe (As-doped >10¹⁹ cm−³) Or can be Or GeSn, Sn content 0 ≤ Sn % ≤ 20%graded Thickness range 100 Å-5000 Å Ge-GeSn for field enhancement 4 P⁻Collector ~10000 Å Ge_(1-x)(Si_(0.8)Sn_(0.2))_(x) (B-doped~1 × 10¹⁶cm⁻³) 5 P⁺ Sub-Collector ~5000 Å SiGe (B-doped~5 × 10¹⁸ cm⁻³) SiGe Gecontent can be varied to accomodate the Ge_(1-x)(Si_(0.8)Sn_(0.2))_(x)layer 6 P⁺ Buffer ~500 Å Si (B-doped~2 × 10¹⁸ cm⁻³) 7 P⁺ Si substrateCrystalline

FIG. 51 shows a possible exemplary flat band energy band diagram for asymmetric double heterojunction transistor where a Ge QW or QD isembedded in the GeSiSn base region with SiGe emitter/cladding and SiGecollector/cladding. Note a GeSn QW or QD can also be used here. Thisstructure can work as an NPN or PNP transistor light emitter device.Here a Ge QW or QD 5103 is inserted into aGe_(1-x)(Si_(0.8)Sn_(0.2))_(x) 5102 & 5104 base region. Note a GeSn QWor QD can replace the Ge QW or QD. The SiGe 5101 emitter/cladding andthe SiGe 5105 collector/cladding, form the major index difference forlight confinement in the waveguide 5106 region, and also have a largebandgap energy to funnel carriers into the active region.

Table 10 shows a possible exemplary structure for a symmetric NPN doubleheterojunction transistor laser structure with an N⁻ SiGeemitter/cladding, a Ge QW or QD embedded in P⁺ GeSiSn base with an N⁻SiGe collector/cladding. Note a GeSn QW or QD can replace the Ge QW orQD.

TABLE 10 A symmetric NPN double heterojunction transistor laserstructure with a SiGe emitter/cladding, a Ge QW or QD embedded in GeSiSnbase with a SiGe collector/cladding. Layer Layer Name DescriptionComment 1 N⁺ Cap ~1000 Å SiGe (As-doped >10¹⁹ cm⁻³) 2 N⁻ EmitterCladding ~4000 Å SiGe (As-doped~5 × 10¹⁷) 3 P⁺ Base ~500 ÅGe_(1-x)(Si_(0.8)Sn_(0.2))_(x) (B-doped >10¹⁹ cm⁻³) 4 QW or QD ~55 Å GeLight emission Or a GeSn, Sn content can be: 0 ≤ Sn % ≤ 20% 1000 nm-5000nm. QW thickness range 10 Å-1000 Å GeSn QW or QD QD size range 10 Å-200Å can be used here. 5 P⁺ Base ~500 Å Ge_(1-x)(Si_(0.8)Sn_(0.2))_(x)(B-doped >10¹⁹ cm⁻³) 6 N⁻ Collector/Cladding ~4000 Å SiGe (As-doped~5 ×10¹⁷ cm⁻³) SiGe Ge content can be varied to accomodate theGe_(1-x)(Si_(0.8)Sn_(0.2))_(x) layer 7 N⁺ sub-collector ~500 Å Si(As-doped~5 × 10¹⁸ cm⁻³) 8 N⁺ Si conducting substrate Crystalline

Table 11 shows a possible exemplary structure for a symmetric PNP doubleheterojunction transistor laser structure with a P⁻ SiGeemitter/cladding, Ge QW or QD embedded in N⁺ GeSiSn base with a P⁻ SiGecollector/cladding. Note a GeSn QW or QD can replace the Ge QD or QW.

TABLE 11 A symmetric PNP double heterojunction transistor laser withSiGe emitter/cladding, Ge QW or QD embedded in GeSiSn base, and SiGecollector/cladding. Layer Layer Name Description Comment 1 P⁺ Cap ~1000Å SiGe (B-doped >10¹⁹ cm⁻³) 2 P⁻ upper Cladding ~4000 Å SiGe (B doped~5× 10¹⁷) 3 N⁺ Base ~500 Å Ge_(1-x)(Si_(0.8)Sn_(0.2))_(x) (As-doped >10¹⁹cm⁻³) 4 QW or QD ~55 Å Ge Light emission Or a GeSn, Sn content can be: 0≤ Sn % ≤ 20% 1000 nm-5000 nm. QW thickness range 10 Å-1000 Å GeSn QW orQD QD size range 10 Å-200 Å can be used here 5 N⁺ Base ~500 ÅGe_(1-x)(Si_(0.8)Sn_(0.2))_(x) (As doped >10¹⁹ cm⁻³) 6 P⁻Collector/Cladding ~4000 Å SiGe (B-doped~5 × 10¹⁷ cm⁻³) SiGe Ge contentcan be varied to accomodate the Ge_(1-x)(Si_(0.8)Sn_(0.2))_(x) layer 7P⁺ Sub-Collector ~500 Å Si (B-doped~5 × 10¹⁸ cm⁻³) 8 P⁺ Si substrateCrystalline

It should be noted that there are many types of N-type and P-typedopants. For standard III-V semiconductors like GaAs, InP, InGaAs,InGaP, the N-type dopants are Si, Ge, Sn, Pb, S, Se, Te. The P-typedopants for standard III-V semiconductors are C, Zn, Be, Mg. Commondopants for group IV semiconductors like GeSn, Ge, Si, SiGe, GeSiSn forN⁻ type dopants are P, As, Sb. The P-type dopants are B, Al, Ga.

Exemplary Configuration 4: Si Emitter—SiGe base with Ge QD or QW—SiCollector transistor laser. The introduction of a Ge QD or QW or (GeSnQD or QW) into a standard SiGe HBT design allows for the noveldevelopment of a Si photonic transistor laser. SiGe has a wide range ofbandgaps from a starting point of Si with a bandgap energy of 1.1 eV, atSi_(0.8)Ge_(0.2) has a bandgap energy of approximately 1 eV, atSi_(0.6)Ge_(0.4) has a bandgap energy of approximately 0.93 eV, and atSi_(0.2)Ge_(0.8) has a bandgap energy of approximately 0.87 eV. Tofabricate a light emitting bipolar transistor the flat band diagram isshown in FIG. 52 for an NPN device. Inserted into the SiGe base is a GeQD or QW or (or GeSn QD or QW). FIG. 52 shows an exemplary flat bandenergy diagram of the material structure.

FIG. 52 shows an exemplary flat band energy diagram of a Si Emitter—SiGebase with Ge QD or QW—Si Collector light emitting HBT 5200. This HBTlaser is grown on Si substrates, thus compatible with Si processing.Here a Ge QD or QW 5204 has barriers region of Si_(0.6)Ge_(0.4) 5203 &5205 P⁺ base/barrier. The Si_(0.6)Ge_(0.4) (note other compositions ofSiGe can be used) forms the P⁺ base and also acts as a barrier layer forquantum confine the electrons and holes in the Ge QD or QW 5204. For aQD the growth of a large lattice constant material on a smaller latticeconstant material results in strained layer epitaxy allowing theself-assembled three dimensional island growth. Typical quantum dotdiameters are in the range of 1-20 nm, but are dependent on thewavelength of light that needs to be emitted. For a growth of the QW, itis typically grown on a lattice matched layer with a larger bandgaplayer than the QW material or can be grown strained. A large bandgapbarrier then covers the QW layer. Typical QW thicknesses are in therange of 5-20 nm, but are not restricted to these thicknesses. The Ge QDor QW 5204 inserted into a base/barrier serves for the collection regionfor electrons and holes to recombine to generate light. TheSi_(0.6)Ge_(0.4) 5203 & 5205 also serve as the optical confinement layerand the waveguide material. The Si 5202 & 5206 serves as the N⁻emitter/cladding and N⁻ collector/cladding material for this structure.The cladding serves as funneling carriers into the active/waveguideregion and traps the emit light in the waveguide 5207 structure.

FIG. 53 shows a possible cross-sectional device depiction of a Si basededge emitting transistor laser or light emitting structure. Thetransistor laser includes a Ge QD or QW 5304 (a GeSn QD or QW can alsobe used) inserted into a Si_(0.8)Ge_(0.2) 5303 & 5305 P⁺ base/barrier ofthe HBT. Layer 5301 is the highly conductive N⁺ type Si contact. Thelaser can require a resonant cavity to get optical gain, and typicallythis can be formed from the front and back cleaved facets of thesemiconductor crystal wafer. The structure can be grown on N⁺ Siconducting substrate 5308, which is the seed crystal to grow the fullstructure. An N⁺ Si sub-collector 5307 is grown on the substrate. An N⁻Si collector/cladding 5306 and the N⁻ Si emitter/cladding 5302 do dualfunctions of optical confinement of the light 5309 produced from theactive region Ge QD or QW 5304 and the controlling the flow of electronsand holes. The P⁺ Si_(0.8)Ge_(0.2) Base 5305 & 5303 form the barriermaterial for the Ge QD or QW 5304, and also provide the waveguidematerial. The laser can require a resonant cavity to get optical gain,and typically this can be formed from the front cleaved facets 5311 andback cleaved facets 5310 of the semiconductor crystalline structure.

Table 12 shows an exemplary structure that could be grown. Note for thisHBT device the Si_(0.8)Ge_(0.2) base could be graded down to lower Sicontent.

TABLE 12 Epitaxial structure of NPN light emitting SiGe-Ge-SiGe HBT.Layer Layer Name Description Comment 1 N⁺ Cap ~2000 Å Si (As-doped >10¹⁹cm⁻³) As = Arsenic 2 N⁻ Emitter/Cladding ~5000 Å Si (As-doped~5 × 10¹⁷cm⁻³) 3 P⁺ Base ~500 Å Si_(0.8)Ge_(0.2) (B-doped >10¹⁹ cm⁻³) SiGe couldbe graded 4 QD or QW Ge Light emission Or GeSn, Sn % can be: 0 ≤ Sn % ≤20% 1000 nm-5000 nm. QD size range~10 Å-500 Å GeSn QD or QW QW thicknessrange~10 Å-1000 Å can be used here. 5 P⁺ Base ~500 Å Si_(0.8)Ge_(0.2)(B-doped >10¹⁹ cm⁻³) SiGe could be graded 6 N⁻ Collector/Cladding ~5000Å Si (As-doped~5 × 10¹⁷ cm⁻³) 7 N⁺ Sub-Collector ~2000 Å Si (As-doped~5× 10¹⁸ cm⁻³) 8 N⁺ Si conducting substrate Crystalline

FIG. 54 shows a laser structure, an additional variation of FIG. 52,because using higher Ge content in the Si_(0.6)Ge_(0.4), it can bepossible to produce QDs or QWs with longer wavelength light emission.FIG. 54 shows the exemplary flat band energy diagram of this structure.The laser includes a Ge QD or QW 5404 inserted into a Si_(0.6)Ge_(0.4)P⁺ barrier/base 5403 & 5405. This HBT laser is grown on Si substrates,thus compatible with Si processing. The Si_(0.6)Ge_(0.4) forms the P⁺base and also acts as a barrier layer for quantum confinement of theelectrons and holes in the Ge QD or QW. The Ge QD or QW 5404 insertedinto a base/barrier serves for the collection region for electrons andholes to recombine to generate light. The Si_(0.6)Ge_(0.4) 5403 & 5405also serve as the optical confinement layer and the waveguide material.The Si_(0.8)Ge_(0.2) 5402 & 5406 serves as the emitter/cladding andcollector/cladding material for this structure. The cladding serves asfunneling carriers into the active/waveguide 5407 region and traps theemitted light in the waveguide structure.

Table 13 shows an exemplary structure that could be grown which includesSi_(0.6)Ge_(0.4) P⁺ base.

TABLE 13 Epitaxial structure of NPN light emitting SiGe-Ge-SiGe HBT(with Si_(.6)Ge_(.4)) Layer Layer Name Description Comment 1 N⁺ Cap~2000 Å Si_(0.8)Ge_(0.2) (As-doped >10¹⁹ cm⁻³) As = Arsenic 2 N⁻Emitter/Cladding ~5000 Å Si_(0.8)Ge_(0.2) (As-doped~5 × 10¹⁷ cm⁻³) 3 P⁺Base ~500 Å Si_(0.6)Ge_(0.4) (B-doped >10¹⁹ cm⁻³) SiGe could be graded 4QD Or QW ~55 Å Ge Light emission Or GeSn, Sn % can be: 0 ≤ Sn % ≤ 20%1000 nm-5000 nm QW thickness range 10 Å-1000 Å GeSn QD or QW QD sizerange 10 Å-200 Å can be used here 5 P⁺ Base ~500 Å Si_(0.6)Ge_(0.4)(B-doped >10¹⁹ cm⁻³) SiGe could be graded 6 N⁻ Collector/Cladding ~5000Å Si_(0.8)Ge_(0.2) (As-doped~5 × 10¹⁷ cm⁻³) 7 N⁺ Sub-Collector ~2000 ÅSi (As-doped~5 × 10¹⁸ cm⁻³) 8 N⁺ Si conducting substrate Crystalline

The transistor laser has the integrated features of both the transistorand the laser. From such a structure it can be straightforward tosimplify the structure and grow a separate confinement heterostructure(SCH) laser.

FIG. 55 shows an exemplary flat band energy diagram of an SCH laserutilizing a Ge QW or QD region 5503 located in UIDGe_(1-x)(Si_(0.8)Sn_(0.2))_(x) barrier/OCL layer 5502 & 5504 region withP⁺ SiGe 5501 cladding and N⁺ SiGe 5505 cladding layers. This structurerepresents a PN junction or diode with an unintentionally doped (UID)active region and optical confinement region between the P⁺ SiGe 5501and N⁺ SiGe 5505 cladding. The UID Ge_(1-x)(Si_(0.8)Sn_(0.2))_(x) 5502 &5504 forms the barrier material for the Ge QW or QD 5503 active region.The combination of the barrier and active region forms the waveguide5506 of the laser. The P⁺ SiGe 5501 cladding region serves for injectionof the holes and for the optical confinement of the light emitted fromthe active region. The N⁺ SiGe 5505 cladding region serves for injectionof the electrons and for the optical confinement of the light emittedfrom the active region in the waveguide 5506. The UIDGe_(1-x)(Si_(0.8)Sn_(0.2))_(x) 5502 & 5504 could be replaced with a Gebarrier layer as other examples of the SCH laser structure. The Gebarrier to GeSn would have a type I heterojunction alignment.

Table 14 shows an exemplary epitaxial structure SCH injection diodelaser with a Ge QW or QD region with GeSiSn barriers.

TABLE 14 Exemplary epitaxial structure SCH injection diode laser with aGe QW or QD region with GeSiSn barriers. Layer Layer Name DescriptionComment 1 P⁺ Cap ~1000 Å SiGe (B-doped >10¹⁹ cm⁻³) 2 P⁺ Cladding ~5000 ÅSiGe (B-doped~1 × 10¹⁸) 3 UID Barrier/OCL ~500 ÅGe_(1-x)(Si_(0.8)Sn_(0.2))_(x) Also could use a Ge barrier 4 QW or QD~55 Å Ge Light emission Or GeSn, Sn % can be: 0 ≤ Sn % ≤ 20% 1000nm-5000 nm. QW thickness range 10 Å-1000 Å A GeSn QW or QD QD size range10 Å-200 Å can be used here. 5 UID Barrier/OCL ~500 ÅGe_(1-x)(Si_(0.8)Sn_(0.2)) Also could use a Ge barrier 6 N⁺ Cladding~5000 Å SiGe (As-doped~1 × 10¹⁸ cm⁻³) SiGe: Ge content can be varied toaccomodate the Ge_(1-x)(Si_(0.8)Sn_(0.2))_(x) layer 7 N⁺ Buffer ~5000 ÅSi (As-doped~5 × 10¹⁸ cm⁻³) 8 N⁺ Si conducting substrate Crystalline

Exemplary Configuration 5A: An NPN GaAs Emitter—GeSn Base—GaN Collectordouble heterojunction bipolar transistor with dissimilar materials. Thisdevice configuration comprises an emitter/base stack of GaAs—GeSn waferbonded to a GaN collector. GaN with its high bandgap offers tremendousimprovements in the breakdown voltage of the HBT. The device elucidatedcan include a double heterojunction GaAs—GeSn—GaN HBT device. The adventof device technology based on GaN with its high electric field strengthis a new direction for high-power RF amplification. GaN based materialshave a large bandgap and high electron saturation velocity. Theembodiments described herein demonstrate a new semiconductor transistorintegrated circuit with ultra-high performance in applications requiringboth high speed and high power rugged electronics. In examples describedherein, the GaN can be grown on the various substrates like sapphire,SiC, Si GaAs, GaN, and template substrates.

Polar GaN wurtzite structure can be grown on sapphire, SiC (manypolytypes: 3C, 4H, 6H, etc.), Si substrates, or template substrates andhas piezoelectric and polarization charge. GaN grown in the wurtzite(hexagonal) phase results in large spontaneous and piezoelectricpolarization charge.

Non-polar GaN cubic (FCC) structure can be grown on GaAs, Si, ortemplate substrates. GaN in this form has no polarization charge. Acubic form of GaN with (001) orientation can be grown on zinc blendGaAs. Thus the cubic GaN can be grown on conducting GaAs which can actas the sub-collector. The zinc-blend (cubic) GaN collector has anegligible conduction band offset with respect to the GeSn base. Theconduction band offset between GaAs and cubic GaN is roughlyΔE_(C)˜−0.024 eV. Thus the GeSn (close to Ge) the conduction band offsetto GaN is about ΔE_(C)˜0 eV at the base/collector heterojunction.Additionally, non-polar wurtzite forms can be cut from the c-planegrowth along the “a” or “m” plane directions. If the GaN is grown alongthe “m” or “a” plane axis, these polarization effects can be eliminated.Typical GaN wurzite crystals grown along the direction (c-plane) ofIII-nitrides suffer from polarization induced electric fields. Electricfields do not exist across the along nonpolar directions (a-plane orm-plane). Thus, high quality non-polar GaN substrate crystals areproduced by slicing a c-plane GaN boule along the “a” or “m” plane. Sucha material in low defect density non-polar substrates have improvedsubstrates for fabrication of devices.

The GaAs—GeSn—GaN heterojunction transistor described herein representsa revolutionary jump in both high power and high frequency performance.This device embodies enormous RF power output, ruggedness, highbandwidth, and good linearity, combined with low turn-on voltage, whichis desirable for minimizing power consumption. This unique arrangementof materials combines the high transconductance of heterojunctionbipolar transistor (HBT) technology, with the enormous breakdown voltage(using a GaN collector), and a desirable emitter-base heterojunction(wide bandgap GaAs emitter on a narrow bandgap high conductivity P-typeGeSn base). The huge breakdown field of GaN allows the use of shortcollector devices with high bandwidths (e.g. cut-off frequency F_(t) andmaximum oscillation frequency F_(max)>than 150 Ghz). The combination ofa low bandgap (<0.66 eV) GeSn base coupled with a wide bandgap GaN (˜3.4eV) collector can be used for high speed power applications. By using avertical stack of junctions, the device layers are shorter, resulting inlower resistances and shorter transit delays, both contributing to muchhigher frequencies. The use of efficient GaAs—GeSn—GaN transistors cansignificantly enhance battery life while also enabling operation at highpowers with exceptional frequency response. Ultra high performancetransistors that can operate at higher temperatures, higher powerdensities, higher voltages and higher frequencies are desirable fornext-generation commercial applications (IT, consumer, automotive,industrial, telecommunications, wireless devices, etc.).

By utilizing various crystal growth technologies, pulse laser ablationepitaxy, molecular beam epitaxy, metal organic chemical vapordeposition, liquid phase epitaxy, vapor phase epitaxy, or various otherepitaxial growth techniques for the growth of base-emitter stack of P⁺GeSn base onto the N⁻ GaAs emitter, thus forming the base-emitter stackis optimized, because at low Sn % lattice of GeSn is close to that ofGaAs. Then the GeSn—GaAs emitter stack can be coupled with the exemplarywafer bonding technology, which then can be merged to the GaN collectoras described herein, thus forming a monolithicGaAs(emitter)-GeSn(base)-GaN(collector) semiconductor stack that is adesirable HBT embodiment for high-power, high-frequency electronics canbe created. In some examples, the uniqueness of embodiments can resultin a near zero conduction band offset through the three differentsemiconductor materials (GaAs—GeSn—GaN). New materials are required tobuild high power electronics that can also operate at frequencies in the10 to 100 GHz range. The formation of near lattice-matched GeSn on GaAsthen wafer bonded to GaN is a possible key to the realization of thesedevices.

FIG. 56 shows the energy bandgaps of various semiconductors vs. theirlattice constant. The graph has a vertical axis with the values of thebandgap energy (eV) and a horizontal axis with the values of the Latticeconstant (Å). Various semiconductors are plotted as a function of theirbandgap energy and lattice constant. The condition of lattice matchingconstrains certain combinations of semiconductors. It is readily seenthat new types of semiconductor devices can be formed if the latticematching constraint were eliminated. It would then be possible tooptimize new types of heterojunctions based on the optimized materialscharacteristics instead of those constrained to near lattice constantmaterials.

The merging of the GeSn base region with the GaN collector by utilizingthe wafer bonding process for fabrication of heterogeneous materialsdescribed herein. With this approach, the GeSn and GaN epitaxial layerscan be joined to make a single composite structure. Monolithic waferbonding is an advanced process for forming PN junctions. This waferbonding technique allows formation of a robust monolithic structure,where the interface is covalently bonded. The new composite materialestablishes the GeSn—GaN base-collector heterointerface. Wafer bondingallows for the formation of a heterointerface without having to performheteroepitaxy of two poorly lattice matched materials.

The new HBT has a base-collector junction comprising the GeSn P⁺ baseregion wafer bonded to the GaN N⁻ collector is described herein. GeSnlattice constant can vary from 5.65 Å to 5.833 and lattice constant ofGaN is 4.4 Å, which is a huge mismatch (>28%). Such a mismatch does notallow for single or unstrained crystal structures, because the criticalthickness for the base to be grown on the collector would be thin. Withour approach, the GeSn and GaN epitaxial layers can be joined to make asingle composite crystalline structure. The wafer bonding techniquedescribed herein allows us to form a junction that is a robustmonolithic structure, where the interface is covalently bonded.

FIG. 57 shows the exemplary wafer bonding process that enablesmonolithic joining of two dissimilar semiconductor materials. Byemploying pressure, heat, gas ambient, and time, covalently bondedcomposite structures can be developed. The new composite materialestablishes the critical GeSn—GaN base-collector heterointerface. Waferbonding allows formation of a heterointerface without having to performheteroepitaxy of two poorly matched materials. Exemplary wafer bondingmethodology comprises step 5701 where the GeSn 5704 and GaN 5705semiconductors are cleaned in preparation for joining, step 5702 theGeSn 5704 and GaN 5705 are placed on each other in between the waferbonder top plate 5706 and wafer bonder bottom plated 5707, basically thejaws of the wafer bonder, and held under heat 5709 and pressure 5708 forthe requisite time and in a gas ambient, then step 5703 the finalstructure is a monolithic composite material with GeSn 5704 bonded toGaN 5705. The GaAs—GeSn—GaN material structure avoids the use of ternaryalloy semiconductors thereby making the epitaxial process less complexand eliminating alloy scattering of electrons.

NPN GaAs—GeSn—GaN HBTs can include the following concepts: Growth ofnear lattice matched P-type GeSn base on N-type GaAs emitter (GeSn/GaAsstack) because the lattice constant of Ge is almost the same as GaAsthus for low content Sn, GeSn has a slightly larger lattice constantthan GaAs. Monolithic formation by wafer bonding of GeSn/GaAs stack tothe N-type GaN (to circumvent large lattice mismatched growth). One ofthe advantages of the embodiments described herein is the formation of aunique transistor semiconductor stack that can have a near zeroconduction band offset between all three materials, with each materialoptimized for overall HBT performance.

FIG. 58 shows an exemplary flat band energy diagram wafer bonded NPNGaAs—GeSn—GaN HBT 5800. Here an emitter up emitter-base stack 5805comprising of N⁻ emitter GaAs 5802 grown on P⁺ Base GeSn 5803 structure.The full monolithic structure can be formed using an epitaxial liftoff(ELO) procedure. This emitter-base stack 5805 is wafer bonded to the N⁻collector GaN 5804 thus forming a wafer bonded junction 5801 at thebase-collector interface. Possible methodologies of wafer bonding thisemitter-base stack to the GaN is described in Exemplary ELO WaferBonding Configuration 6A and in Exemplary Inverted Wafer BondingConfiguration 6B. Note the conduction band offset ΔE_(C) isapproximately near zero through the NPN HBT structure.

Additionally, the base can be graded from Ge—GeSn to have electric fieldenhancement of the charge carriers. FIG. 59 shows an exemplary flat bandenergy diagram of the NPN GaAs, graded Ge—GeSn, GaN HBT 5900. Here abase up emitter-base stack 5905 comprises P⁺ Base compositionally gradedGe—GeSn 5903 and grown on an N⁻ emitter GaAs 5902 structure. Thecompositionally graded Ge—GeSn 5903 layer may comprise at the emitterinterface a Ge or a low Sn % GeSn layer which is graded to higher Sn %GeSn at the collector interface. The compositional grading range can gofrom Ge at the emitter to GeSn at various compositions up to 20%. Thisemitter-base stack 5905 is then wafer bonded with the P⁺ Base next tothe N⁻ collector GaN 5804 thus forming a wafer bonded junction 5901 atthe base-collector interface. Due to the compositional grading of theGe—GeSn 5903 in the P⁺ Base, there is a field enhancement region 5906that accelerates the carriers toward the collector. The methodology ofwafer bonding this emitter-base stack is described in Exemplary InvertedWafer Bonding Configuration 6B. Note the conduction band offset ΔE_(C)is small through the NPN HBT structure.

FIG. 60 shows an exemplary cross-sectional device depiction of the waferbonded GaAs—GeSn—GaN NPN double HBT 6000 in a mesa configuration. Notethat this is a vertical device, which is desirable for powerapplications because the lateral area can be minimized. The device isgrown on an N⁺ SiC 4H substrate 6006. The NPN HBT comprises an emitterbase stack 6001 with a wafer bond 6010 to the GaN structure 6002 to formthe monolithic device. The GaN structure 6002 can comprise a variety offorms, but for an exemplary case the GaN is grown on a SiC substrate,though a GaN, sapphire, GaAs, Si substrate could also be used. Startingwith an N⁺ SiC substrate 6006, which can be of the 3C, 4H, 6H, etc.variety, on which an N⁺ SiC buffer 6007 is grown. Then an N⁺ SiCsub-collector 6008 can be grown, onto which an N⁻ GaN collector 6009 isgrown. This finalizes the GaN structure 6002. Because the GaN is grownon 4H SiC (wurtzite or hexagonal), such a growth results in a polar GaNcollector 6009. For the emitter-base stack 6001 an ELO procedure to bedescribed in Exemplary Configuration 6A, forms the N⁺ GaAs contact6003—the N⁻ GaAs emitter 6004 and the P⁺ GeSn base 6005, finalizing theemitter base stack 6001. P⁺ GeSn base 6005 layer forms the heavily dopedP-type base. GeSn also has a large hole mobility which is a preconditionfor making the base region thin. The conduction band offset betweenGaAs—GeSn is almost zero, thus the majority of the 0.75 eV bandgapdifference appears in the valence band. The GaN can serve as thecollector layer with a large breakdown voltage for the transistorbecause it has a large bandgap energy of 3.2 eV.

Table 15 is an exemplary Epitaxial structure of an NPN GaAs—GeSn—GaNwafer bonded HBT. In this structure, the GaN is grown on SiC which is atypical substrate for the growth and one of the many describedsubstrates that can be used. SiC is preferred for high power electronicsbecause it has the highest thermal conductivity between GaN, Si, GaAsand sapphire the other primary substrates for GaN.

TABLE 15 Epitaxial structure of an NPN GaAs-GeSn-GaN wafer bonded HBT.Layer Layer Name Description Comment 1 N⁺ Cap ~1000 Å InGaAs (Te-doped>10¹⁹ cm⁻³) 2 N⁻ Emitter Cap ~1500 Å GaAs (Si-doped~5 × 10¹⁸ cm⁻³) 3 N⁻Emitter ~500 Å GaAs (Si-doped~3 × 10¹⁷ cm⁻³) 4 P⁺ Base ~500 Å GeSn(B-doped >10¹⁹ cm⁻³) Or graded Ge-GeSn 0 ≤ Sn % ≤ 20% Thickness range100 Å-5000 Å 5 N⁻ Collector ~10000 Å GaN (N-doped~1 × 10¹⁶ cm⁻³) Waferbonded to above 6 N⁺ Sub-Collector ~5000 Å SiC (N-doped~5 × 10¹⁸ cm⁻³) 7High Purity Buffer ~500 Å SiC (N-doped~5 × 10¹⁸ cm⁻³) N = nitrogen 8 N⁺SiC (4H) conducting substrate Crystalline

FIG. 61 shows another possible exemplary cross-section device depictionof the wafer bonded GaAs—GeSn—GaN/Si NPN double HBT 6100 in a mesaconfiguration. Note that this is a vertical device, which is desirablefor power applications because the lateral area can be minimized. Inthis case the device is grown on a face centered cubic (FCC) N⁺ Sisubstrate 6106. The NPN HBT comprises an emitter base stack 6001 with awafer bond 6110 to the GaN structure 6102 to form the monolithic device.The GaN structure 6102 can comprise a variety of forms, but for anexemplary case the GaN is grown on a Si substrate, though a GaN,sapphire, SiC, GaAs substrate could also be used. Starting with an N⁺ Sisubstrate 6106, which an N⁺ Si buffer 6107 is grown. Then an N⁺ Sisub-collector 6108 can be added, onto which an N⁻ GaN collector 6109 isgrown. This finalizes the GaN structure 6102. Because the GaN is grownon FCC Si (cubic) such a growth results in a non-polar GaN collector6109. An ELO procedure in Exemplary Configuration 6A, describes how theemitter-base stack 6001 is wafer bonded to the GaN collector structure.P⁺ GeSn base 6005 layer forms the heavily doped P-type base. GeSn alsohas a large hole mobility which is a precondition for making the baseregion thin. The conduction band offset between GaAs—GeSn is almostzero, thus the majority of the 0.75 eV bandgap difference appears in thevalence band. The GaN can serve as the collector layer with a largebreakdown voltage for the transistor because it has a large bandgapenergy of 3.2 eV.

In some examples, formation of this monolithic composite material of anNPN GaAs—GeSn—GaN wafer bonded HBT can create a desirable devicearchitecture in that the conduction band offsets are near zero for bothemitter-base and base-collector hetero-interfaces, and the valence bandoffset is large at the emitter-base GaAs—GeSn and base— collectorGeSn—GaN heterojunctions. This property allows for the formation ofheterojunction transistor structure that can have large gain (largevalence band offset between GaAs and GeSn), low base sheet resistance(GeSn has high hole mobility), low turn-on voltage (GeSn has low bandgapenergy), and large breakdown voltage (GaN has large breakdown electricfield strength and high saturated velocity), which are desirable devicemetrics for next-generation electronic transistors. The GaAs—GeSn—GaNHBT has the gain of GaAs, the huge breakdown voltage for robustness, thehigh frequency performance greater than GaAs HBTs, the low turn-onvoltage of GeSn, and improved electron transport because of the nearzero conduction band offset between the emitter-base-collector.Electrons can easily be injected from the GaAs emitter through the GeSnbase to the GaN collector. By adding the ability to grade the basecomposition from Ge to GeSn such that the bandgap energy of the materialis gradually reduced throughout the base as described in ourconfigurations. This grading causes an electric field, which in turnreduces the transit time, thus increasing F_(t). The GaAs—GeSn—GaNmaterials stack can be desirable for making NPN HBTs that can outperformstandard SiGe, GaAs, and InP heterojunction bipolar transistors.

Exemplary GaAs Emitter Advantages: The large valence band offset betweenGaAs emitter and GeSn base can stop back injection of holes into theemitter. This is desirable because the base is doped heavily P-type(typically >1×10¹⁹ cm⁻³), with such high doping of the base, theemitter-base valence band offset blocks the holes even though the basedoping is much higher than the N-type emitter doping (low 10¹⁷ cm⁻³).This allows for low N-type doping of the emitter and high P-type dopingof the base, thus lowering base emitter capacitance while stillachieving sizable current gain. GeSn is near lattice matched to GaAs(5.65 Å), which enables dislocation free growth. GaAs—GeSn—GaN materialstructure avoids the use of ternary alloy semiconductors thereby makingthe epitaxial process less complex and eliminating alloy scattering ofelectrons. The use of a GaAs (instead of InGaP) emitter and GaN (insteadof GaAs) collector significantly increases the overall thermalconductivity of the material structure. The GaAs—GeSn emitter basejunction has a large valence (at least 0.72 eV which is larger than theGeSn band gap). This eliminates the back injection of holes to theemitter from the base, which reduces the gain of the transistor.

Exemplary GeSn Base Advantages (low Sn % similar properties to Ge): GeSnhas a low bandgap (lower than Ge: the term low energy bandgap basetypically refers to the relevant semiconductors with bandgaps less than0.75 eV, like GeSn, Ge, InGaAs, and GaAsSb) which results in low turn-onvoltage (less than 0.5 V). GeSn (low Sn %) hole mobility is high (2000cm²/Vs) like Ge and acceptors can be incorporated to high density(>1×10¹⁹ cm⁻³), thus the base can be made ultra-thin while maintaining alow base sheet resistance which increases current gain and decreaseselectron transit time. By adding the ability to grade the basecomposition from Ge to GeSn such that the bandgap energy of the materialis gradually reduced throughout the base as described in ourconfigurations. This grading causes an electric field, which in turnreduces the transit time, thus increasing F_(t). GeSn can be made tobecome a direct gap semiconductor (unlike Ge which is an indirectsemiconductor). GeSn for low Sn concentration has shallow acceptors, sothe hole concentration is generally equal to the acceptor doping leveland independent of temperature. GeSn can be heavily doped P-type. Thelow base sheet resistance (<<less than 18 Ohm/sq) results in a highF_(max). (>than 150 GHz), GeSn hole resistivity should be on the orderof 0.0002 Ohm-cm. The surface recombination velocity is low for P-typeGe and GeSn.

Exemplary GaN Collector Advantages: GaN collector can be grown on GaN,SiC (many polytypes, i.e., 3C, 4H, 6H), Si, Sapphire, GaAs, and templatesubstrates. Thus the substrate can be chosen to optimize the propertiesof the device. For instance for high power devices it can be useful togrow the GaN on SiC substrates because they have a thermal conductivity.Typically GaN comes in the wurtzite and cubic phase. GaN (wurzite andcubic form) has a large lattice mismatch with GeSn, thus wafer bondingcircumvents the problem of growing strained and incompatible layers. GaNhas high breakdown field, which is excellent for the collector breakdownvoltage. GaN has near zero conduction band offset with GaAs, thus noblocking field at the interface. The use of GaN (instead of GaAs)collector significantly increases the overall thermal conductivity ofthe material structure. GaN has a high saturation velocity, thuselectrons travel without intervalley scattering.

Furthermore, because GeSn has a low resistivity of 0.0002 ohm-cm, onecan decrease the thickness of the base significantly, while stillmoderately increasing the base sheet resistance value. The frequencyresponse of the device is related to the F_(t) and F_(max). Therelationship between transit frequency F_(t) and the maximum oscillationfrequency F_(max) is as follows for an HBT:F_(max)=(F_(t)/8πR_(B)C_(CB))^(1/2). The transit frequency F_(t) isbasically the inverse of the time for the electron to traverse theemitter, base and collector. The parameters R_(B) and C_(CB) refer tothe base sheet resistance and the capacitance of the collector basejunction. The parameter F_(max) is the unity power gain frequency andindicates the maximum frequency with power gain from a device. Thetransit frequency can be further improved by having a higher saturationvelocity which the GaN collector exhibits.

The NPN GaAs—GeSn—GaN HBT (referred to as GeSn HBT) as compared tostandard NPN InGaP—GaAs—GaAs HBT (referred as GaAs HBT) would have thefollowing advantages. The differences between the two devices aretypically independent of the emitter, and mostly rely on the basematerial GeSn and the collector material GaN.

Advantage 1: If the GaAs HBT had a base thickness of 1000 Å, the GeSnHBT base thickness could be halved to 500 Å, and the F_(t) for GeSn HBTwould increase because of the thinner base. Because the GeSn baseresistivity (0.0002 ohm-cm) is 10 times less than GaAs resistivity(0.002 ohm-cm), the parameter F_(max) would increase by a factor of(5*F_(t))^(1/2). This is because F_(t) increased because the basethickness was halved, but the base sheet resistance of the GeSn baseonly increased by a factor of two, but it is still 5 times less than thebase sheet resistance of the GaAs HBT. Furthermore, by using GaN(1.5×10⁵ m/s) which has a higher saturation velocity than GaAs (1×10⁵m/s), such a material results in faster electron transit time across theGaN collector.

A commonly used metric for comparing various semiconductors is theJohnson's figure of merit (FOM), which compares different semiconductorsfor suitability for high frequency power transistor applications. Table16 shows a comparison Johnson FOM for Si, GaAs, and GaN.

TABLE 16 Johnson FOM for Si, GaAs, and GaN. Si GaAs GaN Energy Bandgap(eV) E_(gap) 1.1 1.4 3.4 Electron Saturation Velocity (cm/s) 1 × 10⁷   1× 10⁷ 1.5 × 10⁷ V_(sat) Peak Electron Saturation Velocity 1 × 10⁷ 1.9 ×10⁷ 2.5 × 10⁷ (cm/s) V_(sat peak) Normalized Johnson Figure of Merit 19.5 572 (E_(gap) ⁴ × V_(sat peak) ²) Johnson Figure of Merit = Maximumpower times frequency ≈ E_(gap) ⁴ × V_(sat) ²

Advantage 2: Because GaN has such large breakdown voltage for examplefor a Ge—GaN base collector junction where the GaN collector is only5000 Å thick, the breakdown voltage is greater than 100 V. A typicalGaAs heterojunction base-collector, where the GaAs collector is over10000 Å thick has a breakdown voltage of only 20 volts. Thus one canreduce the GaN collector thickness as compared to the GaAs, withouthurting robustness, thus the transit time or F_(t) is increased becauseof a thinner base and collector, which in turn increases the F_(max)with an additional factor because of the low resistivity GeSn base.Typical values of F_(t) and F_(max) should be greater than 150 GHz.

Exemplary Wafer Bonding of GeSn Stack to GaN: The method of waferbonding is chosen as the most direct means of forming the GeSn to theGaN structure. Our pneumatic bonder can eliminate the problemsassociated with the more conventional torqued jig fixtures. Using themethod described here, the bonder allows gradual pressure applicationfor the delicate bonding of GeSn and GaN. The large size heaters in theplates provide fast temperature ramp up for the bonding process. Thebonder has a self-leveling action to the surface mechanism and ensuresthat it is flat with the surface. We have developed different waferbonders and as well as different wafer bonding processes. QuantTera'scustom wafer bonders have two independent temperature controllers toprecisely control the temperature of the top and bottom bonding plates.

Our wafer bonders have a unique feature in that the top and bottomplates are under electronically controlled differential air pressure.There is no non-linear return spring force needing to be concerned. Thetop plate moving up and down relies on the differential air pressure inthe top plate's air cylinder; and thus, the bonding pressure can becontinuously adjusted precisely to provide optimized wafer bondingconditions. Our wafer bonding system has precise temperature andpressure control to ensure the bonding of the materials. Operation step1 comprises lowering the wafer bonder top plate so that it barelytouches the materials to be bonded. Pressure is then slowly applied atthis time and the temperature of the bonder top and bottom plate areraised. Independent temperature control of the top and bottom platetemperatures allows the accommodation of materials that may havedifferent thermal expansion coefficients, thus minimizing stress to thebonded interface. Our bonders can reach temperatures above 500° C. invarious gas ambient, but typically a nitrogen purge is used during thebonding process. Our bonders can accommodate up to 4″ diameter wafers.In a single step we can easily achieve a PN homojunction orheterojunction bonded materials.

FIG. 62 shows QuantTera's pneumatic wafer bonder configuration 6200. Thebonder uses differential air pressure between P1 pressure 6201 and P2pressure 6202, where the pressure is measured by the differentialpressure gauge 6203. The pressure controls the action of the top plate6204 in moving down to clamp the device and substrate 6207, which sitson the bottom plate 6205, which has a ball bearing 6206 for conformalleveling action. Two independent temperature controllers control thetemperature of the top plate 6204 and bottom plate 6205.

Table 17 shows an exemplary wafer bonding process.

TABLE 17 Exemplary Wafer Bonding Process. Step Description 1 The waferis cleaved to appropriate size. 2 Semiconductor materials are thoroughlycleaned. 3 Oxides are removed from surface by chemical etch or plasmaetcher. 4 GeSn stack material and the GaN material are placed on top ofeach other. 5 GeSn and GaN materials are placed in wafer bonder, whichunder pressure joins the materials together at typical temperatures of300-600° C. for 1 to 2 hours. In a gas ambient. 6 The compositestructure is cooled and then removed. 7 The composite unit acts as amonolithic PN structure. 8 Current voltage testing of the PN junction. 9Shear test to see the strength of wafer bonded junction.

Wafer bonding allows formation of a heterointerface without having toperform heteroepitaxy of two poorly matched materials. FIG. 63 shows thecurrent—voltage characteristic of the wafer bonded P GeSn to N− GaNshowing PN rectifying behavior. The vertical axis is current in units ofmA, and the horizontal axis is voltage in units of V. The turn-onvoltage of the device is less than 0.5 V.

The wafer bonding process allows for independent optimization ofmaterials without regard to lattice matching. It should be noted that Gelattice constant is about 5.65 Å and GaN is 4.4 Å, which is a hugemismatch (28%). Interface defects can be minimized by varying waferbonding parameters such as oxide removal, temperature, time, andpressure. Table 18 lists the thermal expansion coefficients of the GaAs,GeSn, and GaN. Because the thermal expansion coefficients of all thematerials are similar, the thermal stress generated during wafer bondingshould be minimal.

TABLE 18 Thermal Expansion Coefficients of GaAs, GeSn, and GaN. ThermalExpansion Coefficient Material (10⁻⁶ K⁻¹) @ 300 K GaAs 6.0 GeSn similarto Ge 5.9 GaN 5.6

Possible advantages of the GaAs—GeSn—GaN HBT devices described herein:The GaAs—GeSn—GaN stack minimizes the conduction band offsets, whichhinder electron transport (ultra-fast transistor action). Thesemiconductor materials are optimized for performance: GaN collector;GeSn base; and GaAs emitter. GaAs and GeSn are near lattice matched andwafer bonding allows for the integration of GaN without having toperform lattice-mismatch growth (Ge is lattice matched to GaAs, thusGeSn for low content Sn has a slightly larger lattice constant thanGaAs). Wafer bonding is desirable for GaAs—GeSn—GaN because the thermalexpansion coefficients are close to each other. The GaAs—GeSn—GaN HBTexceeds SiGe, InP and GaAs HBTs in terms of turn-on voltage, has muchlower base sheet resistance allowing for a much thinner base, and thebreakdown field that is 10 times higher. Two fundamental obstacles toconventional GaN HBTs are the high resistivity and large bandgap energyof the base layer. Such a low bandgap material results in a HBT withhigh base sheet resistivity and large turn-on voltage. Most of the GaNNPN HBTs have utilized complex re-growth strategies in an attempt toaddress these problems. Despite limited success with regard to DCtransistor properties, these issues remain as impediments to highfrequency operation of conventional GaN HBTs. The GaAs—GeSn—GaN HBTdescribed herein solves all these issues and outperform the technologyof traditional systems. Prominent commercial markets exist where theGaAs—GeSn—GaN transistor described herein can be implemented: 1)wireless devices, mobile, and cellular handset market; and 2) RF highpower electronics.

Exemplary Configuration 5B: NPN InGaP Emitter—GeSn Base—GaN CollectorDouble HBT 6400 with all dissimilar materials Desirable combination ofsemiconductors (near Zero Conduction Band Offset betweenEmitter-Base-Collector). To further improve on Configuration 5A, anInGaP emitter region is added that is lattice matched to GaAs. Thisdevice comprises an emitter stack of InGaP—GeSn wafer bonded to a GaNcollector. GaN with it high band gap offers tremendous improvements inthe breakdown voltage of the HBT. Note the InGaP layer can becompositionally graded to enhanced device performance. The monolithicInGaP—GeSn—GaN stack is unusual in that the conduction band offset isnear zero. This special property allows for the formation ofheterojunction transistor structure that can have large gain, and largebreakdown voltage (GaN has large breakdown electric field strength andhigh saturated velocity). These material characteristics can make adesirable bipolar transistor. The InGaP—GeSn—GaN materials stack can bedesirable for making NPN HBTs that can outperform standard SiGe, GaAs,and InP heterojunction bipolar transistors.

FIG. 64 shows the exemplary flat band energy band diagram of NPN InGaPEmitter—GeSn Base—GaN Collector Double HBT 6400. This new materialstructure with near zero conduction band offsets between interfaces anda large valence band offset at the emitter-base and base collectorheterojunction. Electrons can easily be injected from the InGaP emitterthrough the GeSn base to the GaN collector. Conduction band offsets atemitter-base and base-collector junctions are near zero, with largevalence band offsets between the InGaP—GaAs and GaAs—GaNheterojunctions. The band alignments are desirable for high performanceHBTs. Here an emitter up emitter-base stack 6405 comprising N⁻ emitterordered InGaP 6402 grown on P⁺ Base GeSn 5803 structure. The orderedInGaP 6402 should have the smallest conduction band offset ΔE_(C) withthe GeSn 5803. This emitter-base stack 6405 is then wafer bonded to theN⁻ collector GaN 5804 thus forming a wafer bonded junction 6401 at thebase-collector interface. The methodology of wafer bonding thisemitter-base stack is described in Exemplary ELO Wafer BondingConfiguration 6A. Note the conduction band offset ΔE_(C) isapproximately near zero through the NPN HBT structure.

InGaP semiconductor can be grown epitaxially and lattice matched to GaAsat the composition In_(0.49)Ga_(0.51)P. If typically grown at hightemperatures, it can grow in an ordered phase where the crystallinestructure forms sheets of In—P and Ga—P atoms can alternate in the (001)planes of the FCC unit cell without the intermixing of the Ga and Inatoms on the lattice planes. The ordered InGaP results in an almost zeroconduction band discontinuity between the InGaP and GaAs and is calledthe ordered phase (this can be of weakly type I or weakly type IIbecause it is close to zero). With different growth conditions, the Inand Ga atoms can intermix and the disordered InGaP phase can form, whichhas a conduction band offset (0.1 eV vs. 0.03 eV for the ordered phase).In either case the conduction band offset of InGaP to GaAs or GeSn issmall.

Additionally, the base can be graded from Ge—GeSn to have electric fieldenhancement of the charge carriers. FIG. 65 shows an exemplary flat bandenergy diagram of the NPN InGaP-graded Ge to GeSn—GaN HBT 6500. Here forvariation a base up emitter-base stack 6505 comprising P⁺ Basecompositionally graded Ge—GeSn 6503 grown on a N⁻ emitter disorderedInGaP 6502 structure. The compositionally graded Ge—GeSn 6503 layer maycomprise at the emitter a Ge or a low Sn % GeSn layer which is graded tohigher Sn % GeSn at the collector interface. The compositional gradingrange can go from Ge at the emitter to GeSn at various compositions upto 20%. This emitter-base stack 6505 is then wafer bonded with the P⁺Base next to the N⁻ collector GaN 5804 thus forming a wafer bondedjunction 6501 at the base-collector interface. Due to the compositionalgrading of the Ge—GeSn 6503 in the P⁺ Base, there is a field enhancementregion 6506 that accelerates the carriers toward the collector. Themethodology of wafer bonding the emitter-base stack to the GaN isdescribed in Exemplary Inverted Wafer Bonding Configuration 6B. Note theconduction band offset ΔE_(S) is small through the NPN HBT structure.Note the differences between flat band energy diagrams FIG. 63 with theordered InGaP and FIG. 64 with disordered InGaP is small.

Exemplary InGaP (In_(0.49)Ga_(0.51)P) Emitter Advantages: The largevalence band offset between InGaP emitter and GaAs base stops backinjection of holes into the emitter. This allows for low N-type dopingof the emitter and high P-type doping of the base, thus lowering baseemitter capacitance while still achieving sizable current gain. The nearzero band conduction offset between the InGaP and the GeSn base can bedesirable for electron injection into the base layer. Ordered InGaP mayhave reduced temperature sensitivity to the current gain. InGaP can belattice matched to GaAs or Ge or GeSn (low Sn %), which enablesdislocation free growth.

Table 19 shows an exemplary epitaxial structure of NPN InGaP—GeSn—GaNHBT grown and wafer bonded. In this structure the GaN is wurtzitehexagonal structure, “a” or “m” plane material. GaN in this form has nopolarization charge that degrades the base-collector performance.Non-polar GaN wurzite substrates are illustrated here though one coulduse SiC, GaAs, Si, sapphire (non-polar and polar forms). GaN (FCC) canalso be grown on GaAs which also lacks polarization charge effects.

TABLE 19 Epitaxial Structure of NPN InGaP-GeSn-GaN HBT. Layer Layer NameDescription Comment 1 N⁺ Cap ~1000 Å InGaAs (Te-doped > 10¹⁹ cm⁻³) 2 N−Emitter Cap ~1500 Å GaAs (Si-doped~5 × l0¹⁸ cm⁻³) 3 N− Emitter ~500 ÅInGaP (Si-doped~3 × 10¹⁷ cm⁻³) Ordered or disordered 4 P⁺ Base ~500 ÅGeSn (B-doped > 10¹⁹ cm⁻³) Or graded 0 ≤ Sn% ≤ 20% Ge-GeSn Thicknessrange 100 Å-5000 Å 5 N− Collector ~10000 Å Non-polar GaN (Si-doped~1 ×10¹⁶ Wafer bonded cm⁻³) to above 6 N⁺ ~5000 Å Non-polar GaN (Si-doped~5× 10¹⁸ Sub-Collector cm⁻³) 7 Substrate Non-polar GaN N⁺ conductingsubstrate Crystalline

Exemplary Advantages of InGaP—GeSn—GaN HBT Technology: The NPNInGaP—GeSn—GaN stack minimizes the conduction band offsets, which hinderelectron transport (ultra-fast transistor action). The semiconductormaterials are optimized for performance: GaN collector, GeSn base, InGaPemitter. Wafer bonding allows for the integration of GaN without havingto perform lattice-mismatch growth. Wafer bonding is desirable forGaN—GeSn because the thermal expansion coefficients are close to eachother. Additionally, strain effects can be incorporated in this devicebecause the alloy composition of the InGaP can be changed to introducetensile or compressive strain.

GaN collector for its high saturation velocity and large bandgap energywhich results in a high breakdown voltage. In examples described herein,the GaN can be grown on the various substrates like sapphire, SiC, SiGaAs, GaN, and template substrates. Polar GaN wurtzite structure can begrown on sapphire, SiC (many polytypes: 3C, 4H, 6H, etc.), Sisubstrates, or template substrates and has piezoelectric andpolarization charge. GaN is grown in the wurtzite (hexagonal) phaseresults in large spontaneous and piezoelectric polarization charge thuspossibly creating a potential energy barrier at the wafer-bonded Ge—GaNbase/collector interface. Non-polar GaN cubic (FCC) structure can begrown on GaAs, Si, or template substrates. GaN in this form has nopolarization charge that degrades the base-collector performance. Acubic form of GaN with (001) orientation can be grown on zinc-blendeGaAs. Thus the cubic GaN can be grown on conducting GaAs which can actas the sub-collector. The zinc-blende (cubic) GaN collector has anegligible conduction band offset with respect to the GeSn base. Theconduction band offset between GaAs and cubic GaN is roughlyΔE_(C)˜−0.024 eV. Thus the GeSn (close to Ge) conduction band offset toGaN is about ΔE_(C)˜0 eV at the base/collector heterojunction.Additionally, non-polar wurtzite forms, are cut from the c-plane growthalong the “a” or “m” plane directions. If the GaN is grown along the “m”or “a” plane axis, these polarization effects can be eliminated. TypicalGaN wurzite crystals grown along the direction (c-plane) of III-nitridessuffer from polarization induced electric fields. Electric fields do notexist across nonpolar directions (a-plane or m-plane). Thus, highquality non-polar GaN substrate crystals are produced by slicing ac-plane GaN boule along the “a” or “m” plane. Such a material results inlow defect density non-polar substrates, which have improved substratesfor fabrication of devices. Because Configuration 5A showed the waferbonding of emitter stack to a polar GaN collector, for thisConfiguration 5B a non-polar GaN substrate is demonstrated. Finally tofully fabricate the device, the GaAs top half of HBT stack is waferbonded to the GaN.

The (ordered or disordered) InGaP—GeSn emitter base junction has a largevalence offset (at least 1.1 eV which is much larger than the GeSnbandgap) and a small conduction band offset. This eliminates the backinjection of holes to the emitter from the base, which reduces the gainof the transistor. Also because this is a double HBT, the offset voltagein the output characteristic will be reduced thus enhancing the poweradded efficiency. The base is doped heavily P⁺ (typically >1×10¹⁹ cm⁻³),with such high doping of the base, the emitter valence band offsetblocks the holes even though the base doping is much higher than the N⁻emitter doping (low 10¹⁷ cm⁻³). Furthermore, because GeSn has a lowresistivity of 0.0002 ohm-cm, one can decrease the thickness of the basesignificantly, while still moderately increasing the base sheetresistance value. The frequency response of the device is related to theF_(t) and F_(max). The relationship between transit frequency F_(t) andthe maximum oscillation frequency F_(max) is as follows for an HBT:F_(max)=(F_(t)/8πR_(B)C_(CB))^(1/2). The transit frequency F_(t) isbasically inverse of the time for the electron to traverse the emitter,base and collector. The parameters R_(B) and C_(CB) refer to the basesheet resistance and the capacitance of the collector base junction. Theparameter F_(max) is the unity power gain frequency and indicates themaximum frequency with power gain from a device. The transit frequencycan be further improved by having a higher saturation velocity which theGaN collector exhibits.

In some examples, a feature described herein can be the formation of anadvanced manufacturing platform to demonstrate a fully optimizedtransistor semiconductor stack, which cannot be grown with standardcrystal growth methodologies. The uniqueness of the device describedherein lies in the zero conduction band offset through the threedifferent semiconductor materials (InGaP—GeSn—GaN)emitter-base-collector optimized for overall HBT performance, which isimpossible to grow by standard crystal growth techniques. The parametersthat InGaP—GeSn—GaN NPN transistor can achieve are the following: doubleheterojunction, emitter-base, and base collector can reduce offsetvoltage; high gain (large valence band offset at emitter base junction);high breakdown voltages for improved ruggedness for high powerapplications; and a short collector structure can result in improvedelectron transit time.

Exemplary ELO Wafer Bonding Configuration 6A: Fabrication of InGaPEmitter—GeSn Base—GaN Collector double HBT as an example of the ELOwafer bonding device fabrication process. For Configuration 6A anepitaxial lift off (ELO) process and wafer bonding can be used tofabricate the emitter/base stack to the GaN collector. Epitaxial liftoff and wafer bonding process is a quick-turn method for integration offabricated GeSn devices to be joined on the GaN substrate. Combining thetechniques of epitaxial lift off and wafer bonding releases therestrictions of lattice matching imposed by epitaxial growth and opensnew degrees of freedom for the design of semiconductor devices, becausethe combination of unique properties of different materials becomespossible.

FIG. 66 shows a schematic methodology of the epitaxial lift off process6600. An epitaxial layer stack top HBT 6608 (pre-processed top half ofthe HBT device: InGaP emitter/GeSn base stack) is grown on a sacrificialGaAs 6606 substrate with a thin aluminum arsenide (AlAs 6607) insertedin between the two layers. The top HBT 6608 is covered with wax 6609 formechanical strength. This thin AlAs separation layer 6607 is removed byetching in hydrofluoric acid (HF etch AlAs 6610) in order to lift offthe epitaxial layers from the GaAs 6606 substrate. The wax 6609protecting the top HBT 6608 without the GaAs 6606 substrate is thentransferred onto a new substrate like GaN 6611 via Van der Waals forces.This technique allow for the clean and flat surfaces of two dissimilarmaterials to be brought into close proximity where attractive forcespull them together, forming an intimate contact between differentmaterials. The strength of the adhesion depends on the type ofinteraction. Van der Waals forces provide the first step of attraction.The bonding strength can be increased in the materials by wafer bondingat elevated temperatures. Both InP and GaAs devices integrated have beenfabricated with near perfect interfaces for bonding to Si, AlN,Sapphire, and LiNbO₃ (a piezoelectric for SAW applications) wafers. FIG.66 which demonstrates an exemplary epitaxial lift off (ELO) process 6600can be described as follows: (6601) epitaxial HBT stack layer growthwith AlAs separation layer 6607, top HBT 6608 on GaAs 6606 with AlAs6607; (6602) Wax 6609 covers top HBT 6608, epitaxial lift off by HF etchAlAs 6610 removes AlAs 6607 and releases the top layer off of GaAs 6606substrate; (6603) Van der Waals bonding by surface tension of the topHBT 6608 to GaN 6611 substrate; (6604) removal of wax from top HBT 6608on GaN 6611; and (6605) then wafer bonding to further strengthen the topHBT/GaN monolithic structure.

Exemplary details of device fabrication and growth of ELO top half(example: InGaP—GeSn—GaN HBT). FIG. 67 shows the schematic of the tophalf of the HBT InGaP emitter/GeSn base stack 6701 with the inclusion ofthe AlAs separation layer 6703. Note the separation layer could beAlGaAs from 40% to 100% Al. The top half of the HBT InGaP emitter/GeSnbase stack 6701 comprises: a sacrificial GaAs substrate 6702; AlAsseparation layer 6703; P⁺ GeSn Base 6704; N⁻ InGaP emitter 6705; N⁺ GaAscontact 6706 epitaxial stack. This top half of the HBT InGaPemitter/GeSn base stack 6701 will then be wafer bonded to the GaNcollector stack 6707. The GaN collector stack 6707 comprises a startingN⁺ SiC 4H substrate 6708, with an N⁺ SiC sub-collector 6709, thenfinally an N⁻ GaN collector 6710. There could be many differentvariations of the GaN collector stack 6707, such as growth on GaN, Si,GaAs, sapphire substrates, or template substrates.

Table 20 shows an exemplary structure top half of the HBT InGaPemitter/GeSn base stack 6701.

TABLE 20 Exemplary structure top half of the HBT InGaP emitter/GeSn basestack 6701. Layer Layer Name Description Comment 1 N⁺ Cap ~1,000 ÅInGaAs (Te-doped > 10¹⁹ cm⁻³) (non-alloyed) 2 N− Emitter Cap ~1500 ÅGaAs (Si-doped~5 × l0¹⁸ cm⁻³) 3 N− Emitter ~500 Å In_(0.49)Ga_(0.51)P(Si-doped~3 × l0¹⁷ Ordered or cm⁻³) disordered 4 P⁺ Base ~500 Å GeSn(B-doped > 10¹⁹ cm⁻³) Or graded 0 ≤ Sn% ≤ 20 Ge-GeSn Thickness range 100Å-5000 Å 5 Separation ~50 Å AlAs Layer removed 6 High Purity Buffer ~500Å GaAs (un-doped) 7 Sacrificial GaAs substrate Semi-insulating orconducting

Table 21 shows an exemplary GaN collector structure 6707. Note that theGaN collector can be grown on Si, SiC, GaAs, Sapphire, GaN, etc.,substrates. Additionally an exemplary SiC collector structure could alsobe used as shown by the Table 22.

TABLE 21 Exemplary GaN Collector Structure 6707. Layer Layer NameDescription Comment 1 N⁻ Collector ~10,000 Å GaN (Si-doped~1 × 10¹⁶Wurtzite phase cm⁻³) 2 N⁺ Sub-Collector ~5,000 Å 4H SiC (N-doped~5 ×10¹⁸ Standard (nitrogen doped) cm⁻³) 3 N⁺ Buffer ~500 Å 4H SiC 4 4H SiC(conducting substrate) N⁺ substrate: other phases of SiC possible

TABLE 22 Exemplary SiC Collector Structure. Layer Layer Name DescriptionComment 1 Seed adhesion layer or ~50 Å GeSiSn (undoped or p-dopedThickness range electrical performance or n-doped) 10-5000 Å enhancementOr GeSn layer Or Ge Or ZnSe or GaAs or InGaAs, etc. 2 N⁻ Collector~10,000 Å SiC (N-doped~1 × 10¹⁶ nitrogen-doped cm⁻³) 3 N⁺ Sub-Collector~5,000 Å 4H or 6H SiC (N-doped Standard (nitrogen doped) ~5 × 10¹⁸ cm⁻³)4 N⁺ Buffer ~500 Å 4H or 6H SiC 5 4H or 6H SiC (conducting substrate) N⁺substrate: other phases of SiC possible.

From this point the device to wafer bonded can be pre-processed orpost-processed. For this first exemplary configuration the ELO devicewill be demonstrated pre-processed (device has been partiallyfabricated). The pre-processed top half of HBT is covered in a “blackwax” (Apiezon W) or “white wax” (crystal bond) or other type ofadhesive. In some examples, it is useful to place a mechanical holderlike an exemplary sapphire mechanical substrate to the wax foradditional rigidity and a mechanical strength. The separation AlAs layeris undercut in hydrofluoric HF acid and deionized water at roomtemperature at various ratios. After release the etchant is diluted withde-ionized water, the wax-covered ELO structure is moved to the GaNsubstrate where Van der Waals bonding occurs. In various embodiments,the adhesion process is handled in water to minimize contamination ofthe surfaces.

FIG. 68 shows a pre-processed top half of the HBT which comprises the N⁺GaAs contact 6706; N⁻ InGaP emitter 6705; GeSn base 6704; AlAsseparation layer 6703 on GaAs substrate 6702; mesa device structure withemitter contact 6801; and base contact 6802. Next comes the HF etch ofAlAs and ELO 6807 step. Before etching the device, an adhesive like wax6808 is melted on pre-processed fabricated Top Half of HBT 6800 andsometimes it can be useful to have a mechanical substrate 6810 place onthe wax for additional mechanical strength. This can be useful in largewafer devices (2″, 3″, 4″, 6″, 12″, 18″, etc., wafer sizes and not onlylimited to these sizes). For the ELO process the wax 6808 coatedpre-processed fabricated Top Half of HBT 6800 assembly is placed into asolution of anhydrous hydrofluoric (HF) acid and over time the waxcoated top half of the HBT 6809 is removed from the sacrificial GaAssubstrate 6702. The solution can be heated and temperature controlled,the lift off process time depends on area of the device, and can takefrom several minutes to many hours. Once the sample is lift off, the waxprovides for mechanical strength of the lifted off layer and allows forease of transport to the GaN substrate.

FIG. 69 shows where the top half of HBT wafer bonded to collectorstructure 6901, which comprises the wax coated top of the HBT 6809placed on the GaN collector structure 6707 with van der Waals bonding.Note the GaN collector could have a layer to electrically enhanceperformance, which can be deposited by PLD any other epitaxial process.There are many other seed layers that could be used for this purpose.The seed layer could be used for adhesion or modifying the electricalinterface properties of the heterojunction to improve performance orreliability.

Initially, the wax coated top of the HBT 6809 placed on the GaNcollector structure 6707 with van der Waals bonding results in theadhesion of these layers. The structure can be put intotrichloroethylene or acetone or some solvent to remove the wax, whichthen forms the wafer bonded HBT 6902. To finalize the device for test, abottom metal contact 6903 is applied to the N⁺ SiC 4H substrate 6708.The final wafer bonded structure can then be placed in a wafer bonder,and under heat and pressure, stronger bond formation between the tophalf of the HBT and the GaN collector structure 6707 should result for apermanent final structure. Finally, bottom metallization of thestructure allows for the testing of this heterojunction bipolarstructure for DC testing in a standard emitter-base-collectorconfiguration. For RF testing, contacts can be put on top of thesub-collector to reduce the capacitance effects of the substrate, butthis uses standard RF device fabrication techniques.

Exemplary Inverted Wafer Bonding Configuration 6B: Device Fabrication &Growth of inverted top half of the GeSn base HBT for wafer bonding andpost-processing.

Additionally, it can be useful to use an inverted top of the HBT forwafer bonding. FIG. 70 shows an inverted top half of HBT 7000. Thiscomprises growth on a sacrificial GaAs substrate 7001; followed by alattice matched InGaP etch stop 7002; then an N⁺ contact GaAs 7003; thenan N⁻ GaAs emitter 7004; and then the P⁺ GeSn base 7005. Table 23 showsan exemplary design of the structure.

TABLE 23 Exemplary epitaxial structure of the inverted top half of HBT.Layer Layer Name Description Comment 1 P⁺ Base ~500 Å GeSn (B-doped >10¹⁹ cm⁻³) Or graded Ge-GeSn 0 ≤ Sn% ≤ 20 Thickness range 100 Å-5000 Å 2N⁻ Emitter ~500 Å In_(0.49)Ga_(0.51)P (Si-doped~3 × l0¹⁷ Ordered orcm⁻³) disordered 3 N⁻ Emitter Cap ~1500 Å GaAs (Si-doped~5 × l0¹⁸ cm⁻³)4 N⁺ Contact ~1000 Å GaAs (Si-doped~5 × l0¹⁸ cm⁻³) 5 ~50 Å InGaP StopEtch 6 High Purity ~500 Å GaAs UID Buffer 7 Sacrificial GaAs substrateSemi-insulating or conducting

FIG. 71 shows the straightforward wafer bonding 7101 of the inverted tophalf of HBT 7000 to the GaN collector structure 6707. This step showsthe final wafer bonded structure 7102 and removal of the sacrificialGaAs substrate and InGaP stop etch. The wafer bonded junction occurs atthe P⁺ GeSn base to the N− GaN collector. The wafer bonding processwould occur under heat, pressure, time, current/voltage bias, gaseousenvironment, etc. The final step is the wafer bonded structure andremoval of sacrificial GaAs substrate 7001 and InGaP Stop etch 7002.Thus, after wafer bonding, the sacrificial GaAs substrate 7001 and InGaPetch stop 7002 would be removed by lapping (thinning) and then etchingto the InGaP etch stop layer. This leaves the final epitaxial waferbonded HBT structure 7103.

FIG. 72 shows the post-processing of final epitaxial wafer bonded HBTstructure 7103, where a standard quick lot HBT device fabricationprocedure can be used to test the HBT. The device fabrication of thestructure relies on standard GaAs fabrication techniques because thewafer bonded junction 7204 never gets exposed to any of the chemicalprocesses. This standard device processing technology is used for thefabrication of HBTs and can be generally used for all the differentconfigurations previously elucidated. This HBT test mask set willprovide immediate feedback for fine tuning the growth process HBT devicefabricated using a Quick lot mask set. There are many methodologies forthe fabrication of the HBT. One possible version is to apply photoresistacross the surface of the HBT structure 7103. A photomask will be placedon the photoresist surface and exposed with UV light which imprints apattern of the emitter metal contact 7201 across the surface. Next, theHBT structure 7103 with exposed photoresist is put into a developersolution. The pattern is developed and then metal is blanket coatedacross the surface using a metal evaporator. The whole structure is putinto acetone which dissolves the photoresist leaving only emitter metalcontact 7201 pattern across the top of the N⁺ GaAs contact 7003.

The emitter metal contact 7201 then can act as a metal mask for mesaetching the HBT structure 7103 down to the P⁺ GeSn Base 7005. Here,photoresist is spun all over the mesa etched structure. With a photomaskthat has base metal contact 7202 pattern is placed on the photoresist.The photomask covering the photoresist is subsequently exposed with UVlight and then developed to open a pattern where the base metal contact7202 can be deposited on the P⁺ GeSn base 7005. Typically a metalevaporator will deposit blanket metal all over the surface of the mesaetched structure. The structure will then be put in acetone for metallift off, thus resulting in a pattern of base metal contact 7202 on theP⁺ GeSn Base 7005.

Next, back metallization or the bottom metal contact 7203 is applied tothe N⁺ SiC 4H substrate 6708. FIG. 72 shows the final wafer bonded HBTstructure 7200. Sometimes the HBT device needs to be alloyed at elevatedtemperature to activate the metal contacts. The device is ready for DCtesting.

Typical parameters that are measured and used to qualify the HBTmaterials are sheet resistance of the emitter, base and sub-collector byboth TLM and van Der Pauw cross structures. Various sized HBTs (emittersizes are 40×40, 50×50, 75×75, 100×100 μm²) are used to determineeffects of geometry to device parameters such as Gummel, Gain, OutputCharacteristics and breakdown voltages.

Exemplary Configuration 7: NPN GaAs Emitter—Ge (or GeSn) Base—GaN (orSiC) Collector Double heterojunction with all dissimilar materialshaving Near Zero Conduction Band Offset between Emitter-Base-Collector.GaAs—Ge—GaN heterojunction bipolar transistor (HBT), as describedherein, embodies RF power output, ruggedness, high bandwidth and goodlinearity, and when combined with low turn-on voltage is desirable forminimizing power consumption. The arrangement of materials describedherein combines high transconductance, enormous breakdown voltage (GaNcollector), and a desirable emitter-base heterojunction (wide bandgapGaAs emitter on a narrow bandgap Ge high conductivity p-type base). Thehuge breakdown field of GaN will allow the use of short collectordevices with high bandwidths (cut-off frequency f_(T) and maximumoscillation frequency f_(max)). This HBT will enable significantimprovements in RF power amplifier (PA) efficiency for communicationbase stations.

A combination of semiconductors for ultra-high performance transistorsby utilizing a favorable conduction band alignment between theemitter-base-collector junctions, thus form an optimized heterojunctiontransistor: A Near Zero Conduction Band Offset exists between GaAs(emitter)—Ge (base)—GaN (collector). The P-type Ge base is latticematched to N-type GaAs emitter (Ge/GaAs stack). GeSn could also be usedas the base layer. GaN collector can be grown on different N⁺substrates. Monolithic integration of materials by wafer bonding ofGe/GaAs stack wafer to the N-type GaN collector (circumvents largelattice mismatched growth).

The monolithic GaAs—Ge—GaN stack has a near zero conduction band offset.This property allows for the formation of heterojunction transistorstructure that can have large gain (large valence band offset betweenGaAs and Ge). Additionally, these materials allow for a low base sheetresistance, low turn-on voltage (Ge has high hole mobility and lowbandgap energy), and large breakdown voltage (GaN has large breakdownelectric field strength and high saturated velocity). These materialcharacteristics make for a desirable bipolar transistor. The GaAs—Ge—GaNmaterials stack is desirable for making NPN HBTs that can significantlyoutperform standard high power GaN transistors. FIG. 73 shows anexemplary flat band energy diagram of the NPN GaAs Emitter—Ge (or GeSn)Base—GaN (or SiC) Collector Double heterojunction with all dissimilarmaterials having Near Zero Conduction Band Offset betweenEmitter-Base-Collector. Here an emitter up emitter-base stack 7306comprising of N⁻ emitter GaAs 7301 and P⁺ base Ge 7302 structure. Thefull monolithic structure can be formed using an epitaxial lift off(ELO) procedure. This emitter-base stack 7306 is wafer bonded tocollector stack 7307 thus forming a wafer bonded junction 7310 at thebase-collector interface. The wurtzite N collector GaN 7303 can be grownon 4H or 6H (or other variations) N⁺ sub-collector SiC 7304, andcomprise the collector stack 7307. Possible methodologies of waferbonding this emitter-base stack 7306 to the collector stack 7307 isdescribed in Exemplary ELO Wafer Bonding Configuration 6A and inExemplary Inverted Wafer Bonding Configuration 6B. Note the conductionband offset ΔE_(C) is approximately near zero through the NPN HBTstructure. The band diagram of this new material structure with nearzero conduction band offsets between interfaces and a large valence bandoffset at the emitter-base heterojunction allows for electrons to beeasily injected from the GaAs emitter through the Ge base to the GaNcollector. Note a GeSn base can be used in this configuration.

To form such a structure, the interface between the base-emitter stack7306 and the collector stack 7307 can require wafer bonding, because thelattice constants of the base material and collector material are highlylattice mismatched.

An exemplary structure that could be grown and wafer bonded isillustrated in the following table. Table 24 shows an exemplaryepitaxial structure of an NPN GaAs—Ge-Hexagonal GaN wafer bonded HBT. Inthis structure, the GaN is grown on SiC which is a typical substrate forthe growth and one of the many described substrates that can be used.SiC is preferred for high power electronics because it has the highestthermal conductivity between GaN, Si, GaAs and sapphire the otherprimary substrates for GaN. In this structure, GaN can be grown onwurtzite GaN, 4H or 6H SiC, or sapphire.

TABLE 24 Epitaxial Structure of NPN GaAs-Ge-Hexagonal GaN HBT. LayerLayer Name Description Comment 1 N⁺ Cap ~1000 Å InGaAs (Te-doped > 10¹⁹cm⁻³) Standard (non-alloyed) 2 N⁻ Emitter Cap ~1500 Å GaAs (Si-doped~5 ×10¹⁸ cm⁻³) Standard 3 N⁻ Emitter ~500 Å GaAs (Si-doped~3 × 10¹⁷ cm⁻³)Standard (InGaP can also be used as the emitter) 4 P⁺ Base ~500 Å Ge(B-doped > 10¹⁹ cm⁻³) Thickness range or GeSn, 0 ≤ Sn% ≤ 20 100-5000 Å 5N⁻ Collector ~10000 Å GaN (Si-doped~1 × 10¹⁶ cm⁻³) Wafer bonded to above6 N⁺ Sub-Collector ~5000 Å 4H SiC (N-doped~5 × 10¹⁸ cm⁻³) Standard(nitrogen doped) 7 N⁺ Buffer ~500 Å 4H SiC Standard 8 4H or 6H SiC(conducting substrate) N⁺ Substrate: Excellent thermal conductivity

In other examples, there is a cubic form of GaN that can be used in theHBT device structure. The GaN can be grown face centered cubic (FCC) on3C SiC. GaN in this form can have no polarization charge that degradesthe base-collector performance. GaN (FCC) can also be grown on Sisubstrates or on template substrates that are commercially available.FIG. 74 shows an exemplary flat band energy diagram of the NPN GaAsEmitter—Ge (or GeSn) Base—GaN (or SiC) Collector Double heterojunctionwith all dissimilar materials having Near Zero Conduction Band Offsetbetween Emitter-Base-Collector. Here an emitter up emitter-base stack7306 comprising of N⁻ emitter GaAs 7301 grown on P⁺ base Ge 7302structure. The full monolithic structure can be formed using anepitaxial lift off (ELO) procedure. This emitter-base stack 7306 iswafer bonded to collector stack 7407 thus forming a wafer bondedjunction 7410 at the base-collector interface. The cubic N⁻ collectorGaN 7403 can be grown on 3C (or other variations) N⁺ sub-collector SiC7404 (or an N⁺ sub-collector Si 7405), and can comprise the collectorstack 7407. Possible methodologies of wafer bonding this emitter-basestack 7306 to the collector stack 7407 is described in Exemplary ELOWafer Bonding Configuration 6A and in Exemplary Inverted Wafer BondingConfiguration 6B. Note the conduction band offset ΔE_(C) isapproximately near zero through the NPN HBT structure. The band diagramof this new material structure with near zero conduction band offsetsbetween interfaces and a large valence band offset at the emitter-baseheterojunction allows for electrons to be easily injected from the GaAsemitter through the Ge base to the GaN collector. Note a GeSn base canbe used in this configuration.

To form such a structure the interface between the base-emitter stack7306 and the collector stack 7407 can require wafer bonding, because thelattice constants of the base material and collector material are highlylattice mismatched.

GaN in the cubic form has no polarization charge that degrades thebase-collector performance. GaN (FCC) can also be grown on Si substratesor on template substrates that are commercially available. Table 25shows an exemplary epitaxial structure of an NPN GaAs—Ge-Cubic GaN waferbonded HBT. In this structure, the GaN is grown on SiC which is atypical substrate for the growth and one of the many describedsubstrates that can be used. SiC is preferred for high power electronicsbecause it has the highest thermal conductivity between GaN, Si, GaAsand sapphire the other primary substrates for GaN. In this structure,GaN can be grown on cubic GaN, 3C SiC, Si, or other substrates.

TABLE 25 Epitaxial Structure of NPN GaAs-Ge-Cubic GaN HBT. Layer LayerName Description Comment 1 N⁺ Cap ~1000 Å InGaAs (Te-doped > 10¹⁹ cm⁻³)Standard (non-alloyed) 2 N⁻ Emitter Cap ~1500 Å GaAs (Si-doped~5 × 10¹⁸cm⁻³) Standard 3 N⁻ Emitter ~500 Å GaAs (Si-doped~3 × 10¹⁷ cm⁻³)Standard (InGaP can also be used as the emitter) 4 P⁺ Base ~500 Å Ge(B-doped > 10¹⁹ cm⁻³) Thickness range or GeSn, 0 ≤ Sn% ≤ 20 100-5000 Å 5N⁻ Collector ~10000 Å GaN (Si-doped~1 × 10¹⁶ cm⁻³) Wafer bonded to above6 N⁺ Sub-Collector ~5000 Å 3C SiC (N-doped~5 × l0¹⁸ cm⁻³) Standard(nitrogen doped) 7 N⁺ Buffer ~500 Å 3C SiC Standard 8 3C SiC (conductingsubstrate) N⁺ Substrate: Excellent thermal conductivity

In various embodiments, a thin Ge or GeSn layer can be put on the GaN topromote adhesion of the wafer bonding of the Ge or GeSn to the GaN. Thisthin film can be grown epitaxially by Metalorganic chemical vapordeposition (MOCVD), molecular beam epitaxy (MBE), pulsed laserdeposition (PLD), or other forms of deposition.

The device described herein, a GaAs (emitter)/Ge (base) wafer bonded tofour different varieties of GaN collector structures (GaN/GaN, GaN/SiC,GaN/Sapphire, GaN/Si), should result in an optimized collectorstructure. Ge can be an desirable base layer due to its low bandgapenergy and the fact it has the highest hole mobility of anysemiconductor.

GaAs Emitter Advantages (InGaP can also be used as the emitter withsimilar advantages): The large valence band offset between GaAs emitterand Ge base stops back injection of holes into the emitter. This allowsfor low n-type doping of the emitter and high p-type doping of the base,thus lowering base emitter capacitance while still achieving sizablecurrent gain. Ge is lattice matched to GaAs which enables dislocationfree growth. The use of AlGaAs or disordered or ordered InGaAP emittercould also be used in this device structure.

Ge Base Advantages: Ge has a low bandgap which results in low turn-onvoltage. Ge hole mobility is high and acceptors can be incorporated tohigh density, thus the base can be made ultra-thin while maintaining alow base sheet resistance which increases current gain and decreaseselectron transit time. Ge has true shallow acceptors, so the holeconcentration is generally equal to the acceptor doping level andindependent of temperature. The low base sheet resistance results in ahigh f_(max). The surface recombination velocity is low for p-type Ge.Low resistance ohmic contacts can be formed on p-type Ge.

GaN Collector Advantages: GaN has a large lattice mismatch with Ge, thuswafer bonding circumvents the problem of growing strained andincompatible layers. GaN collector can be grown on: (1) lattice matchedGaN, (2) 4H SiC, 6H SiC, (3) sapphire, (4) 3C SiC (cubic GaN eliminatesthe polarization charge that arises in Wurzite GaN), (5) on Si, or (6)other substrates. GaN has high breakdown field which is excellent forthe collector breakdown voltage. SiC has many polytypes and only a fewhave been listed above. GaN has near zero conduction band offset withGe, thus no blocking field at the interface. GaAs—Ge—GaN materialstructure avoids the use of ternary alloy semiconductors therebyeliminating alloy scattering of electrons. GaN collector significantlyincreases the overall thermal conductivity of the material structure.GaN has a high saturation velocity thus electrons travel withoutintervalley scattering. GaN power maximum capability is 572 timesgreater than that of Si and 60 times greater than that of GaAs.

FIG. 75 shows bandgap energies of various semiconductors as a functionof the lattice constant. It can be readily seen that new types ofsemiconductor devices could be formed if the lattice matchingconstraints were eliminated. It would then be possible to optimizeheterojunctions based on the materials characteristics instead of thoseconstrained to near lattice constant materials. It is with this aim thatmanufacturing wafer bond methodologies can be used to join dissimilarmaterials to form fully optimized state-of-the-art electronic devices.

The low bandgap Ge base can significantly decrease transistor turn-onvoltage and thereby increase the power added efficiency of the device.The GaAs—Ge—GaN structure described herein can have the lowest turn-onvoltage. FIG. 76 shows a graph of the collector current density J_(C)vs. the turn-on voltages V_(BE) of various HBT material systems. Thefigure shows a plot of the collector current density J_(C) (A/cm²)vertical scale versus base-emitter voltage V_(BE) (V) horizontal scale.The plotted characteristics (ideal) for several different heterojunctionbipolar transistor (HBT) technologies are shown. The Ge HBT structuredescribed herein has the lowest turn—on voltage 7601 of the technologiesshown of InP/InGaAs, SiGe, GaAs, and GaN/InGaN.

Summary of Features of GaAs—Ge—GaN HBT materials. The semiconductormaterials can be optimized: GaAs emitter, Ge base, and GaN collector.The GaAs—Ge—GaN stack minimizes the conduction band offsets which hinderelectron transport (ultra fast transistor action). GaAs and Ge are nearlattice matched and wafer bonding allows for the integration of GaNwithout having to perform lattice-mismatch growth. Wafer bonding isdesirable for GaAs—Ge—GaN because the thermal expansion coefficients areclose to each other. GaN can be grown on GaN, 4H SiC, 6H SiC, 3C SiC,sapphire, or Si substrates, these include the wurtzite and cubic forms.

The GaAs—Ge—GaN NPN heterojunction materials should achieve thefollowing metrics for next-generation electronic transistors: (1) Lowestturn-on voltage for bipolar materials; (2) High Gain (both large valenceband offset and Ge low resistivity of 0.0002 Ohm-cm, allows the use of athinner base region, which enables current gains greater than GaAs); (3)The thin base also enhances the transit time of the electrons across thebase (large f_(T)) and high frequency of operation f_(max) (lower basesheet resistance=higher f_(max)); and (4) High breakdown voltagesimproves ruggedness and enables higher power applications.

Exemplary Choice of Materials Ge Wafer Bonding. Ge (5.64613 Å) is almostlattice matched to GaAs (5.6533 Å). The wafer bonding of P⁺ Ge (Gadoped) substrates to the GaN collector has been done. The Ge wafers willP⁺ doped greater than 1×10¹⁹ cm⁻³, 4″ diameter wafer, and 140 micronsthick with (100) crystal orientation, but could be (111) orientation andnot only limited to this.

Different GaN collector structures include the following: (1) N⁻ GaN(1×10¹⁷ cm⁻³) on N⁺ GaN (>(1×10¹⁹ cm⁻³) substrate, there is zero latticemismatch in this structure; (2) N⁻ GaN (1×10¹⁷ cm⁻³) on N⁺ 4H SiC(>(1×10¹⁹ cm⁻³) substrate, there is about a 4% lattice mismatch betweenthe layers. Presently, SiC is used as the substrate for GaN epitaxy, 6HSiC or various other polytypes may also work; (3) N⁻ GaN (1×10¹⁷ cm⁻³)on N⁺ GaN (>(1×10¹⁹ cm⁻³) grown on sapphire substrates. There is about a14% lattice mismatch between the two layers; (4) N⁻ GaN (1×10¹⁷ cm⁻³) onN⁺ 3C SiC (>(1×10¹⁹ cm⁻³) substrate, there is about a 4% latticemismatch between the layers; (5) Cubic N⁻ GaN (1×10¹⁷ cm⁻³) on N⁺ Si(>(1×10¹⁹ cm⁻³) substrates; and (6) Other substrate combinations withGaN could be used for demonstration of this device. The methods (1),(2), and (3) result in wurtzite GaN. The use of cubic GaN may eliminatethe polarization charge effects that occur in the wurtzite GaN phase.

Exemplary Configuration 8: NPN GaAs Emitter—Ge (or GeSn) Base—SiCCollector Double heterojunction with all dissimilar materials havingNear Zero Conduction Band Offset between Emitter-Base-Collector. Thisarrangement of materials described herein combines hightransconductance, enormous breakdown voltage (SiC collector), and adesirable emitter-base heterojunction (wide bandgap GaAs emitter on anarrow bandgap Ge high conductivity P-type base, additionally GeSn canbe used as the P-type base). The huge breakdown field of SiC will allowthe use of short collector devices with high bandwidths (cut-offfrequency f_(T) and maximum oscillation frequency f_(max)). The use ofGaAs/Ge/SiC or GaAs/GeSn/SiC transistors can significantly enhancebattery life in wireless devices, cellular, and smartphone applications,while also enabling operation at high powers with exceptional frequencyresponse.

A feature of the device described herein is the formation of aheterojunction bipolar transistor, a desirable combination ofsemiconductors for transistors by utilizing a favorable conduction bandalignment between the emitter-base-collector junctions, thus forming aheterojunction bipolar transistor: (1) A near Zero Conduction BandOffset exists between GaAs (emitter)—Ge (base)—GaN (collector); (2) TheP-type Ge base is lattice matched to N-type GaAs emitter (Ge/GaAsstack); (3) SiC collector structure; and (4) Monolithic integration ofmaterials by wafer bonding of Ge/GaAs stack wafer to the n-type SiCcollector (circumvents large lattice mismatched growth).

The combination of semiconductors GaAs/Ge stack wafer bonded to SiC forhigh performance transistors (Near Zero Conduction Band Offset betweenEmitter-Base-Collector). The monolithic GaAs—Ge—SiC stack has a nearzero conduction band offset. This property allows for the formation ofheterojunction transistor structure that can have large gain (largevalence band offset between GaAs and Ge). Additionally, these materialsallow for a low base sheet resistance, low turn-on voltage (Ge has highhole mobility and low bandgap energy), and large breakdown voltage (SiChas large breakdown electric field strength and high saturatedvelocity). These material characteristics comprise a useful bipolartransistor.

The GaAs—Ge—SiC or GaAs—GeSn—SiC or GaAs-graded Ge to GeSn—SiC materialsstack is useful for making NPN. FIG. 77 shows an exemplary flat bandenergy diagram of GaAs—Ge—SiC stack grown on 4H SiC substrate (energybandgaps are in parenthesis), with near zero conduction band offsetsbetween interfaces and a large valence band offset at the emitter-baseheterojunction. Electrons are easily injected from the GaAs emitterthrough the Ge base to the SiC collector. FIG. 77 shows the conductionband offsets at emitter-base and base-collector junctions are near zero,with large valence band offsets between the GaAs—Ge and Ge—SiCheterojunctions. FIG. 77 shows an exemplary flat band energy diagram ofthe NPN GaAs Emitter—Ge Base—SiC Collector Double heterojunction withall dissimilar materials having Near Zero Conduction Band Offset betweenEmitter-Base-Collector. Here an emitter up emitter-base stack 7306comprising of an N⁻ emitter GaAs 7301 and a P⁺ base Ge 7302 structure.The full monolithic structure can be formed using an epitaxial lift off(ELO) procedure. This emitter-base stack 7306 is wafer bonded tocollector 7703 thus forming a wafer bonded junction 7710 at thebase-collector interface. The wurtzite N⁻ collector 4H SiC 7703 can begrown on 4H or 6H (or other variations) sub-collector/substrates.Possible methodologies of wafer bonding this emitter-base stack 7306 tothe collector 7703 is described in Exemplary ELO Wafer BondingConfiguration 6A and in Exemplary Inverted Wafer Bonding Configuration6B. Note the conduction band offset ΔE_(C) is approximately near zerothrough the NPN HBT structure. The band diagram of this new materialstructure with near zero conduction band offsets between interfaces anda large valence band offset at the emitter-base heterojunction allowsfor electrons to be easily injected from the GaAs emitter through the Gebase to the SiC collector. Note a GeSn base can be used in thisconfiguration.

To form such a structure the interface between the base-emitter stack7306 and the collector 7703 can require wafer bonding, because thelattice constants of the base material and collector material are highlylattice mismatched.

An exemplary structure that could be grown and wafer bonded isillustrated in Table 26.

TABLE 26 Epitaxial Structure of a NPN GaAs-Ge-SiC HBT. Layer Layer NameDescription Comment 1 N⁺ Cap 1000 Å InGaAs (Te-doped > 10¹⁹ cm⁻³)Standard (non-alloyed) 2 N⁻ Emitter Cap 1500 Å GaAs (Si-doped~5 × 10¹⁸cm⁻³) Standard 3 N⁻ Emitter 500 Å GaAs (Si-doped~3 × 10¹⁷ cm⁻³) Standard4 Base 500 Å Ge (p-doped: B > 10¹⁹ cm⁻³) Thickness range or GeSn 0 ≤ Sn%≤ 20 100-5000 Å or graded Ge to GeSn 5 N⁻ Collector 10000 Å SiC(nitrogen-doped~1 × 10¹⁶ Wafer bonded to above cm⁻³) 6 N⁺ Sub-Collector5000 Å 4H SiC (~5 × 10¹⁸ cm⁻³) Standard (nitrogen doped) 7 4H SiC(conducting substrate) N+ Substrate: Excellent thermal conductivity

FIG. 78 shows an exemplary flat band energy diagram of the NPN GaAsEmitter—GeSn Base—SiC Collector Double heterojunction with alldissimilar materials having Near Zero Conduction Band Offset betweenEmitter-Base-Collector (energy bandgaps are in parenthesis). With nearzero conduction band offsets between interfaces and a large valence bandoffset at the emitter-base heterojunction electrons can be easilyinjected from the GaAs emitter through the GeSn base to the SiCcollector. FIG. 78 shows conduction band offsets at emitter-base andbase-collector junctions can be near zero, with large valence bandoffsets between the GaAs—GeSn and GeSn—SiC heterojunctions. Here anemitter up emitter-base stack 7806 comprising of an N⁻ emitter GaAs 7801and a P⁺ base GeSn 7802 structure. The full monolithic structure can beformed using an epitaxial lift off (ELO) procedure. This emitter-basestack 7806 is wafer bonded to collector 7703 thus forming a wafer bondedjunction 7810 at the base-collector interface. The wurtzite N⁻ collector4H SiC 7703 can be grown on 4H or 6H (or other variations)sub-collector/substrates. Possible methodologies of wafer bonding thisemitter-base stack 7806 to the collector 7703 is described in ExemplaryELO Wafer Bonding Configuration 6A and in Exemplary Inverted WaferBonding Configuration 6B. Note the conduction band offset ΔE_(C) can beapproximately near zero through the NPN HBT structure.

To form such a structure the interface between the base-emitter stack7806 and the collector 7703 can require wafer bonding, because thelattice constants of the base material and collector material are highlylattice mismatched.

In other examples, there is a cubic form of SiC that can be used in theHBT device structure. The SiC can be grown face centered cubic (FCC) on3C SiC. SiC in this form can have no polarization charge that degradesthe base-collector performance. SiC (FCC) may also be grown on Sisubstrates or on template substrates that are commercially available.FIG. 79, shows an exemplary flat band energy diagram of GaAs—GeSn—SiCstack grown on 3C SiC (because this would also have a near zeroconduction band offset from the emitter—base—collector—subcollector tothe substrate, energy bandgaps are in parenthesis). Conduction bandoffsets at emitter-base and base-collector junctions are near zero, withlarge valence band offsets between the GaAs—GeSn and GeSn—SiCheterojunctions (note a Ge base could be used). Here an emitter upemitter-base stack 7806 comprising of an N⁻ emitter GaAs 7801 grown onP⁺ Base Ge 7802 structure. The full monolithic structure can be formedusing an epitaxial lift off (ELO) procedure. This emitter-base stack7806 is wafer bonded to SiC collector 7903 thus forming a wafer bondedjunction 7910 at the base-collector interface. Possible methodologies ofwafer bonding this emitter-base stack 7806 to the collector 7903 isdescribed in Exemplary ELO Wafer Bonding Configuration 6A and inExemplary Inverted Wafer Bonding Configuration 6B. Note the conductionband offset ΔE_(C) is approximately near zero through the NPN HBTstructure. The band diagram of this new material structure with nearzero conduction band offsets between interfaces and a large valence bandoffset at the emitter-base heterojunction allows for electrons to beeasily injected from the GaAs emitter through the GeSn base to the SiCcollector. Note a Ge base can be used in this configuration.

To form such a structure the interface between the base-emitter stack7806 and the collector 7903 can require wafer bonding, because thelattice constants of the base material and collector material are highlylattice mismatched.

It can be useful to put a thin GeSiSn or GeSn layer down on the SiC topromote adhesion of the wafer bonding of the Ge or GeSn base. This thinfilm can be done epitaxially by MOCVD, MBE or PLD. Note if a Ge layer isused that layer may also be terminated with a GeSn layer to promoteadhesion to the SiC. This is a possible configuration of the step gradebase (Ge to GeSn) material.

Additionally, the base can be linearly or other possible grading, gradedfrom Ge to GeSn to have electric field enhancement of the chargecarriers (electrons). Such structure creates an electric field thataccelerates the electrons across the base to the collector.Additionally, the base can be graded from Ge—GeSn to have electric fieldenhancement of the charge carriers. FIG. 80 shows an exemplary flat bandenergy diagram of the NPN GaAs emitter—graded Ge to GeSn base—SiCcollector HBT. Here a variation of the base up emitter-base stack 8006comprising a P⁺ base compositionally graded Ge—GeSn 8002 grown on an N⁻emitter GaAs 8001 layer. The compositionally graded Ge—GeSn 8002 layercan comprise at the emitter interface a Ge or a low Sn % GeSn layerwhich is graded to higher Sn % GeSn at the collector interface. Thecompositional grading range can go from Ge at the emitter to GeSn atvarious compositions up to 20%. This emitter-base stack 8006 is thenwafer bonded with the P⁺ base next to the N⁻ collector SiC 8003 thusforming a wafer bonded junction 8010 at the base-collector interface.Due to the compositional grading of the Ge—GeSn 8002 in the P⁺ base,there is a field enhancement region 8011 that accelerates the carrierstoward the collector. The methodology of wafer bonding the emitter-basestack to the SiC is described in Exemplary Inverted Wafer BondingConfiguration 6B. Note the conduction band offset ΔE_(C) is smallthrough the NPN HBT structure. Note the grade can be linear, stepped,parabolic, or any reasonable variation. Also the graded layer can begrown by one technique such as MOCVD or can comprise of multiple growthdeposition such as but not limited to the MOCVD growth of the Ge and thesubsequent PLD growth of the GeSn layer.

GaAs Emitter Advantages: (1) The large valence band offset between GaAsemitter and Ge base stops back injection of holes into the emitter. Thisallows for low n-type doping of the emitter and high p-type doping ofthe base, thus lowering base emitter capacitance while still achievingsizable current gain; (2) Ge is lattice matched to GaAs which enablesdislocation free growth; and (3) The use of AlGaAs or disorderd orordered InGaP emitter could also be used in this device structure.

Ge Base Advantages (similar to GeSn): (1) Ge has a low bandgap whichresults in low turn-on voltage; (2) Ge hole mobility is high andacceptors can be incorporated to high density, thus the base can be madeultra-thin while maintaining a low base sheet resistance which increasescurrent gain and decreases electron transit time; (3) Ge has trueshallow acceptors, so the hole concentration is generally equal to theacceptor doping level and independent of temperature; (4) The low basesheet resistance results in a high f_(max); (5) The surfacerecombination velocity is low for p-type Ge; and (6) Low resistanceohmic contacts can be formed on p-type Ge.

SiC Collector Advantages: (1) SiC has many crystalline polymorphs. Thecommon ones are hexagonal 4H SiC, 6H SiC, and cubic 3C SiC; (2) 3C cubicSiC has no polarization charge; (3) SiC has high breakdown field whichis excellent for the collector breakdown voltage. SiC has near zeroconduction band offset with Ge, thus no blocking field at the interface;(4) GaAs—Ge—SiC or GaAs—GeSn—SiC material structure avoids the use ofternary alloy semiconductors thereby eliminating alloy scattering ofelectrons; (5) SiC collector significantly increases the overall thermalconductivity of the material structure; (6) SiC has a high saturationvelocity thus electrons travel without intervalley scattering; and (7)4H SiC power maximum capability is 286 times greater than that of Si.

Exemplary Wafer Bonding of GeSn or Ge Stack to SiC: The method of waferbonding is chosen as the most direct means of forming the GeSn to theSiC structure. Using the method described here, the bonder allowsgradual pressure application for the delicate bonding of GeSn and SiC.The large size heaters in the plates provide fast temperature ramp upfor the bonding process. The bonder has a self-leveling action to thesurface mechanism and ensures that it is flat with the surface. Also thewafer bonder can be current and voltage biased for anodic wafer bondingor in situ monitoring the current and voltage during the bondingprocess. Table 27 shows basic exemplary wafer bonding process. Also inthe wafer bonding process the top and bottom plates can be biased forvoltage and current to monitor the wafer bonding process to enhance thewafer bonding process (anodic wafer bonding).

TABLE 27 Wafer Bonding Procedure. Step Description 1 The wafer iscleaved to appropriate size. 2 Semiconductor materials are thoroughlycleaned. 3 Oxides are removed by wet etch. HCl is used for Ge or GeSn.HF is used for SiC. Then put into methanol. 4 Ge or GeSn stack materialand the SiC material are placed on top of each other and kept inmethanol until transferred to the wafer bonder. 5 Ge or GeSn stack andSiC materials are placed in wafer bonder, which holds the materialstogether at typical temperatures of 300-600° C. for 15 to 360 minutes(but these can be changed at a pressure between 1 to 10 psi). Typicalambient gas is nitrogen or hydrogen or any other form. Also the top andbottom plate can be current voltage biased for in situ monitoring oranodic wafer bonding. 6 The composite structure is slowly cooled andthen removed. 7 The composite unit acts as a monolithic structure and isready for testing.

The wafer bonding allows for independent optimization of materialswithout regard to lattice matching. It should be noted that GeSn latticeconstant is greater than 5.65 Å and 4H SiC lattice constant is 3.1 Å,which is a huge mismatch.

A commonly used metric for comparing various semiconductors is theJohnson's figure of merit (FOM), which compares different semiconductorsfor suitability for high frequency power transistor applications. Table28 shows a comparison of possible Johnson FOM for Si, GaAs, and GaN.

TABLE 28 Comparison of Energy Bandgap, Electron Saturation Velocitiesand Johnson Figure of Merit for Si, GaAs, and GaN. Si GaAs GaN EnergyBandgap (eV) E_(gap) 1.1 1.4 3.4 Peak Electron Saturation 1 × 10⁷ 1.9 ×10⁷ 2.5 × 10⁷ Velocity (cm/s) V_(sat peak) Normalized Johnson FOM 1 9.5572 Johnson Figure of Merit = Maximum power × frequency = P_(max) × f ≈(E_(gap) ⁴ × V_(sat peak) ²)

Table 29 shows a comparison of possible Johnson FOM for 3C SiC, 4H SiC,and 6H SiC.

TABLE 29 Comparison of Energy Bandgap, Electron Saturation Velocitiesand Johnson Figure of Merit for 3C SiC, 4H SiC and 6H SiC. 3C SiC 4H SiC6H SiC Energy Bandgap (eV) E_(gap) 2.4 3.2 3.0 Peak Electron Saturation2 × 10⁷ 2 × 10⁷ 2 × 10⁷ Velocity (cm/s) V_(sat peak) Normalized JohnsonFigure of Merit 91 286 221 Johnson Figure of Merit = Maximum power ×frequency = P_(max) × f ≈ (E_(gap) ⁴ × V_(sat peak) ²)

Interface defect formation: Thermal expansion coefficients of exemplarymaterials are shown in Table 30. Due to the fact thermal expansioncoefficients of all the materials are similar, the thermal stressgenerated during wafer bonding should be minimal.

TABLE 30 Thermal Expansion Coefficients. Thermal Expansion CoefficientMaterial (10⁻⁶ K⁻¹) @ 300 K GaAs 6.0 Ge 5.9 GaN 5.6 3C SiC 3.8 4H SiC4.3 6H SiC 4.3

Table 31 shows thermal conductivities of the various semiconductors. SiChas high thermal conductivities.

TABLE 31 Thermal Conductivities of Semiconductors. Thermal ConductivityMaterial (Wcm⁻¹ K⁻¹) @ 300K GaAs 0.55 Ge 0.58 GaN 1.3 3C SiC 3.6 4H SiC3.7 6H SiC 4.9

Exemplary Configuration 9: NPN GaAs Emitter—GeSn (or Ge) Base—ZnSeCollector Double heterojunction with all dissimilar materials. Thedevice elucidated in this example can include a asymmetric doubleheterojunction GaAs—GeSn—ZnSe HBT device. This device can have desirablebase characteristics with a low voltage base turn-on (<0.5 V) region anda symmetric heterojunction thus eliminating the offset voltage in thetransistor output characteristic that reduces power added efficiency.FIG. 81 illustrates an exemplary flat band energy diagram of thematerial structure. This device can be desirable for high speed and RF(radio frequency) power amplification. This material is near latticematched thus making it a useful structure for crystal growth techniques.

The monolithic GaAs—GeSn—ZnSe HBT does not have zero conduction bandoffset from emitter to base to collector, however the conduction bandalignment is favorable. This transistor structure can have large gain(large valence band offset between GaAs and GeSn or Ge). Additionally,these materials allow for a low base sheet resistance, low turn-onvoltage (GeSn has high hole mobility and low bandgap energy), and largebreakdown voltage (ZnSe has large breakdown electric field strength andhigh saturated velocity). These material characteristics make for auseful bipolar transistor.

FIG. 81 shows an exemplary flat band energy diagram of the NPN GaAsEmitter—GeSn Base—ZnSe Collector Double heterojunction with electrontransport favorable conduction band offsets between interfaces and alarge valence band offset at the emitter-base heterojunction. Electronsare easily injected from the GaAs emitter through the GeSn base to theZnSe collector. FIG. 81 shows the conduction band offsets atemitter-base junction are near zero, with large valence band offsetsbetween the GaAs—GeSn and GeSn—ZnSe heterojunctions. Here the emittercomprising of an N⁻ GaAs 8101 and a P⁺ base GeSn 8102 structure, with alarge bandgap energy N⁻ collector ZnSe 8103. Note a Ge base can be usedin this configuration. Such a device can be useful for high speed andhigh power applications where a high mobility base and a large breakdowncollector voltage is desired.

FIG. 82 shows an exemplary schematic embodiment of an NPN GaAs—GeSn—ZnSedouble heterojunction bipolar transistor in a mesa configuration. Notethis structure could be grown inverted thus one could start the growthusing GaAs substrates and finish with the ZnSe collector andsub-collector. Note that this can be a vertical device, which can bedesirable for power applications because the lateral area can beminimized. FIG. 82 shows a general configuration of a GaAs—GeSn—ZnSeheterojunction bipolar transistor as a vertical stack geometry.Typically the structure can be grown epitaxially or by various means.For a vertical heterojunction bipolar transistor, typically asemi-insulating ZnSe substrate 8201 is used as the seed crystal to startthe growth of the structure. A highly conducting N⁺ ZnSe sub-collector8202 is then grown, followed by a low doped N⁻ ZnSe collector 8203. A P⁺GeSn base 8204 is then grown, followed by a GaAs emitter 8205, andfinally a highly conducting GaAs contact layer 8206. Electrical contactis made to device via the metalized contact pads: emitter contact 8210,base contact 8211, and collector contact 8212. The voltages and currentsare applied to the device via the contact pads. Vertical configurationoffers some advantages.

Additionally, the base can be linearly or other possible grading, gradedfrom Ge to GeSn to have electric field enhancement of the chargecarriers (electrons). Such structure creates an electric field thataccelerates the electrons across the base to the collector. The gradingwill go from Ge at the emitter to GeSn at various compositions up to20%. A possible method would be to grow the emitter first and then gradethe Ge to GeSn. FIG. 83 shows an exemplary flat band energy diagram ofthe NPN GaAs emitter—graded Ge to GeSn base—ZnSe collector HBT. Thecompositionally graded Ge—GeSn 8302 layer can comprise at the emitterinterface starting with Ge or a low Sn % GeSn layer which is graded tohigher Sn % GeSn at the collector interface. The compositional gradingrange can go from Ge at the emitter to GeSn at various compositions upto 20%. Due to the compositional grading of the Ge—GeSn 8302 in the P⁺base, there is a field enhancement region 8311 that accelerates thecarriers from emitter GaAs 8301 toward the collector ZnSe 8303. Note thegrade can be linear, stepped, parabolic, or any reasonable variation.Also the graded layer can be grown by one technique such as MOCVD or cancomprise multiple growth deposition such as but not limited to the MOCVDgrowth of the Ge and the subsequent PLD growth of the GeSn layer.

Table 32 shows an exemplary structure that could be grown.

TABLE 32 Epitaxial Structure of NPN GaAs—GeSn—ZnSe HBT. Layer Layer NameDescription Comment 1 N⁺ Cap (non- ~1000 Å InGaAs Te = telluriumalloyed) (Te~doped > 10¹⁹ cm⁻³) InGaAs layer is fully relaxed 2 NEmitter Cap ~1500 Å GaAs (Si~doped ~5 × Si = silicon 10¹⁸ cm⁻³) 3 NEmitter ~500 Å GaAs (Si~doped ~3 × 10¹⁷ cm⁻³) 4 P⁺ Base ~500 Å GeSn(p-doped: 0 < Sn% < 20 B > 10¹⁹ cm⁻³) Exemplary or Ge thickness of or Geto graded GeSn 100 Å-5000 Å B = boron 5 N⁻ Collector ~10,000 Å ZnSe(doped Chlorine doped ~1 × 10¹⁶ cm⁻³) 6 N⁺ Sub- ~5000 Å ZnSe (dopedChlorine doped Collector ~5 × 10¹⁸ cm⁻³) 7 High Purity Buffer ~500 ÅZnSe (un-doped) No doping 8 ZnSe semi-insulating substrate

The lattice constants of these materials are shown in Table 33. Notethey are similar thus it is possible to grow this structure by standardcrystal growth processes. For higher Sn % GeSn it can be advantageous togrow the GeSn on the ZnSe collector.

TABLE 33 Lattice Constants. Material Lattice Constants (Å) @ 300K GaAs~5.653 Ge ~5.646 ZnSe ~5.668

Exemplary Configuration 10: Si Emitter—SiGe base with GeSn quantumwell—SiGe Collector—Si sub-collector double light emittingheterojunction transistor laser or LED for Si photonics. Theintroduction of a GeSn quantum well or quantum dot or Ge quantum dotinto a standard SiGe HBT design allows for the novel development of a Siphotonic transistor laser. SiGe has a wide range of bandgaps from astarting point of Si with a bandgap energy of 1.1 eV, atSi_(0.8)Ge_(0.2) has a bandgap energy of approximately 1 eV, atSi_(0.6)Ge_(0.4) has a bandgap energy of approximately 0.93 eV, and atSi_(0.2)Ge_(0.8) has a bandgap energy of approximately 0.87 eV. Tofabricate a light emitting bipolar transistor the following flat banddiagram is shown for an n-p-n device. Inserted into the SiGe base is aGeSn quantum well or quantum dot or Ge quantum dot.

FIG. 84 shows an exemplary flat band energy diagram of a separateconfinement heterostructure (SCH) laser diode device utilizing a GeSn QWor QD region located in UID Ge barrier/OCL layer 8403 & 8405 region withP Si_(0.8)Ge_(0.2) 8402 cladding and N Si_(0.8)Ge_(0.2) 8406 claddinglayers. This structure represents a PN junction or diode with anunintentionally doped (UID) active region and optical confinement regionbetween the P Si_(0.8)Ge_(0.2) 8402 cladding and N Si_(0.8)Ge_(0.2) 8406cladding. The UID Ge 8403 and 8405 forms the barrier material for theGeSn QW or QD 8404 active region. The combination of the barrier andactive region forms the waveguide 8408 of the laser. The P⁺ Si 8401layer serves for injection of the holes into the device. The N⁺ Si 8407layer serves for injection of the electrons into the device. Thecarriers are collected in the waveguide region and recombine in the GeSnQW or QD 8404 active region to generate light. Thus it is called anelectrical injection laser. Though this depicts a symmetric structure itcan be also asymmetric.

A further innovation is to take the SCH laser structure and form atransistor laser structure. FIG. 85 shows an exemplary flat band energydiagram of the transistor laser structure. The transistor laser includesa GeSn QW or QD 8504 active region inserted into a Si_(0.8)Ge_(0.2)P⁺base/barrier 8503 & 8505. This HBT laser is grown on Si substrates, thuscompatible with Si processing. The Si_(0.8)Ge_(0.2) forms the P⁺ baseand also acts as a barrier layer for quantum confinement of theelectrons and holes in the GeSn QW or QD 8504. Additionally theSi_(0.8)Ge_(0.2) 8506 collector can be part of the waveguide 8508. TheGeSn QW or QD 8504 inserted into a base/barrier serves for thecollection region for electrons and holes to recombine to generatelight. The P⁺ base Si_(0.8)Ge_(0.2) 8503 & 8505 and the Si_(0.8)Ge_(0.2)N⁻ collector 8506 serve as the optical confinement layer and thewaveguide 8508 material. The Si emitter cladding 8502 & Si sub-collectorcladding 8507 serve as the cladding layers for this structure. Thecladding serves as funneling carriers into the active waveguide 8508region and traps the emitted light in the waveguide structure.

Table 34 shows an exemplary structure that could be grown for theepitaxial structure of NPN light emitting Si—GeSn—SiGe HBT. Note thebase QW well could also be formed by compression. Note for this HBTdevice the Si_(0.8)Ge_(0.2) base could be graded down to lower Sicontent or be replaced with a Ge base material.

TABLE 34 Epitaxial Structure of NPN light emitting Si-GeSn-SiGe HBT.Layer Layer Name Description Comment 1 N⁺ Cap ~2000 Å Si (As-doped >10¹⁹ cm⁻³) As = Arsenic 2 N⁻ Emitter Contact ~5000 Å Si (As-doped ~5 ×10¹⁸ cm⁻³) Cladding 3 N⁻ Emitter ~1000 Å Si (As-doped ~5 × 10¹⁷ cm⁻³)Cladding Undoped-layer ~50 Å Si_(0.8)Ge_(0.2) Intrinsic layer 4 P⁺ Base~1000 Å Si_(0.8)Ge_(0.2) (p-doped: B > 10¹⁹ SiGe could be cm⁻³) gradedor B = Boron Ge layer could be used QW ~10-1000 Å Ge_(0.9)Sn_(0.1) QW orQD or ~10-500 Å GeSn quantum dot 0 < Sn% < 20 Light emission ~1000-5000nm P⁺ Base ~200 Å Si_(0.8)Ge_(0.2) (p-doped: B > 10¹⁹ SiGe could begraded cm⁻³) B = Boron or Ge layer could be used Undoped-layer ~50 ÅSi_(0.8)Ge_(0.2) Intrinsic layer 5 N Collector ~1200 Å Si_(0.8)Ge_(0.2)(As-doped ~5 × 10¹⁵ cm⁻³) 6 N⁺ Sub-Collector ~6000 Å Si (As-doped ~1 ×10¹⁹ cm⁻³) Cladding 7 N⁺ Buffer ~500 Å Si (As-doped) 8 N⁺ Si conductingsubstrate Note for this structure a variety of compositions of the SiGecan be used (0% to 80% Ge%)

The front and back cleaved facets form the mirror of the laser.Additional anti-reflection coating can be put on the facets to providefor a better resonant cavity. Then metallizing the top and bottom of thetransistor structure with an aperture open in the top or bottom metalwould allow for the light to leave. FIG. 86 shows a possible exemplarycross-sectional device depiction of a Si based edge emitting transistorlaser or light emitting structure. Note in this schematic embodiment theGeSn QW could be replaced by a GeSn quantum dot or Ge quantum dot.Typical quantum dot sizes are 1 to 50 nm. Note the verticalconfiguration could also be possible by putting dielectric orsuperlattice mirrors on top of the Si contact layer and the bottom ofthe Si substrate. The transistor laser includes a GeSn QW 8604 activeregion inserted into a Si_(0.8)Ge_(0.2) 8603 & 8605 P⁺ base/barrier ofthe HBT. The laser can require a resonant cavity to get optical gain,and typically this can be formed from the front 8611 and back cleavedfacets 8610 of the semiconductor crystal wafer. The structure can begrown on N⁺ Si conducting substrate 8608, which is the seed crystal togrow the full structure. An N⁺ Si sub-collector/cladding 8607 is grownon the substrate. An N⁻ Si_(0.8)Ge_(0.2) collector 8606 which also formsa part of the waveguide is grown on the sub-collector. The N⁻ Siemitter/cladding 8602 and the N⁺ Si sub-collector/cladding 8607 do dualfunctions of optical confinement of the light 8609 produced from theactive region GeSn QW 8604 and controlling the flow of electrons andholes into the active region. The P⁺ Si_(0.8)Ge_(0.2) base 8605 & 8603form the barrier material for the GeSn QW 8604, and also provide thewaveguide material. The laser can require a resonant cavity to getoptical gain, and typically this can be formed from the front cleavedfacets 8611 and back cleaved facets 8610 of the semiconductorcrystalline structure. Layer 8600 is the highly conductive N⁺ Si contactlayer. Layer 8601 is the highly conductive N⁺ Si emitter contact layer.

There are atmospheric transmission windows at 2-2.5 μm and 3.4-4.2 μmand this type of transistor laser structure can be useful for developingcost effective Si based photonic devices for telecommunicationsapplications. Additionally such a device can be useful on-chip or chipto chip communications.

Exemplary Configuration 11: Crystal growth modification of interfacesfor wafer bonding. The crystal growing techniques to grow the devicestructure in the patent are well known in the literature. The varioustechniques such as Metalorganic Chemical Vapor Deposition (MOCVD),Molecular beam epitaxy (MBE), Vapor Phase Epitaxy (VPE), Liquid PhaseEpitaxy (LPE), etc., can grow various epitaxial structures and can beused for interface modification of the wafer bonding procedure. Anexemplary description of pulsed laser deposition will be used todescribe modification of interfaces for wafer bonding of two crystals.

Pulsed laser deposition (PLD) epitaxy is a crystal methodology to formsingle layers on a suitable substrate. The system comprises of a targetholder and a substrate holder housed in a vacuum chamber. A pulsedNd-YAG laser, excimer laser, etc., beam is directed toward a sourcetarget, which vaporizes the source (laser ablation), and creates a beamof source particles (plasma plume) for deposition onto heated substrate.

For an exemplary situation for wafer bonding Ge or GeSn to GaN or SiC, athin layer of Ge, GeSn or other materials can be deposited by PLD topromote adhesion and better interface formation on one or both of thelayers to be wafer bonded. Additionally, one could deposit ZnSe tochange the band bending or as a method for neutralizing thepiezoelectric charge that can occur in wurzite GaN or SiCheterostructures.

FIG. 87 shows a possible exemplary method of using PLD deposited layerto promote adhesion and an optimized heterojunction. In this situationthin adhesion layers are applied to both materials but it can also bepossible to apply the adhesion layer to only one of the materials. Theadhesion layers can be the same or different depending on application.SiC has a strong oxide and this is a method to get around that issue andform materials with electrical and adhesion properties that areoptimized for the heterojunction formation. The PLD tool can also beused to deposit quantum wells, quantum dots and all variety of epitaxialstructures on materials that are difficult to grow on.

In this exemplary example, FIG. 87 show GeSn 8705 could be deposited onGe 8707 by PLD, a plasma plume of GeSn will reach the Ge surface andreadily bond to the Ge. Additionally in this exemplary case GeSiSn 8706could be deposited by PLD on the SiC 8708, a plasma plume of GeSiSn willreach the SiC surface and readily bond to the SiC. The Ge 8707 with GeSnat its surface is the first bonding material. The SiC 8708 with GeSiSnat its surface is the second bonding material Step 8700 shows that bothmaterials are oriented with their modified surfaces toward each other.Step 8701 shows that the modified surface Ge 8707 is placed on themodified surface of the SiC 8708, with the modified interfaces incontact. This structure is placed in the wafer bonder with wafer bondertop plate 8710 and wafer bonder bottom plate 8711 clamping thestructure. With application of heat 8712 and pressure/current/voltage8713 and with time the two structures 8707 and 8708 can be wafer bondedtogether. Step 8702 shows the final monolithic structure. Additionallythe conditions of wafer bonding include the amount of pressure betweenthe top and bottom plates of the bonder, biasing the top and bottomplates of the wafer bonder by a voltage, or applying a current throughthe top and bottom plates of the bonder, in addition to gas ambient thatthe bonding process is occurring, and other possible conditions.

Exemplary Configuration 12: GaNInAs materials for heterojunctiondevices. In some embodiments, the GaNInAs heterojunction device caninclude transistors or other types of devices, and can be used as anabsorbing material for solar cells and photodetectors, or as an activeregion of a laser as a quantum well or quantum dot region.

The quaternary material GaNInAs can be considered as a combination ofthe binary materials GaN and InAs. The GaNInAs materials system can beunique because it can be tailored to make a low bandgap of approximately1 eV. The binary constituents can form end points of GaN with a latticeconstant of 3.2 Å and InAs with a lattice constant of 6.06 Å. GaNInAs atvarious compositions can be lattice matched to GaAs, Si, Ge, etc.

In many embodiments, GaNInAs film thicknesses from 1500 Å to 5000 Å canbe deposited (e.g., grown) on GaAs substrates by pulsed laser deposition(PLD). In some embodiments, the thicknesses can depend on growthconditions and deposition time. In further embodiments, the GaNInAs filmcan be deposited on the GaAs substrates approximately 2 degrees offtoward the (110) or (111) plane. In other embodiments, the GaNInAs filmcan be deposited on the GaAs substrates approximately 0 degrees offtoward the (110) or (111) plane. PLD is a physical vapor depositionprocess and can be similar to molecular beam epitaxy, but can be simplerin many examples. With PLD, almost any type of material can be depositedin a vacuum or with a purging gas ambient. Also, various materials canbe co-deposited to form alloy semiconductors by use of multiple targets.PLD can be one of the simplest but most versatile forms of thin filmepitaxial techniques. A pulsed laser beam is directed toward a sourcetarget, which vaporizes the source, and creates a beam of sourceparticles (plasma plume) for deposition onto a heated GaAs substrate orother substrate materials. Other forms of epitaxy such as molecular beamepitaxy, metalorganic vapor deposition, vapor phase epitaxy, liquidphase epitaxy, atomic layer deposition, and other various crystalgrowing techniques can be used to manufacture the GaNInAs film.

A PLD system can be used to grow the GaNInAs films. In theseembodiments, a composite GaN/InAs (P-type) target can be used, which canbe rotated in the pulsed laser beam to co-deposit the materials to formthe quaternary GaNInAs. A ratio of the beam flux between the GaN andInAs can determine the composition of the GaNInAs films. Meanwhile, anInAs single crystal target can be heavily doped which can act as asource of P-type doping for the quaternary film. Using a single crystalGaN and a single crystal InAs can make the quaternary GaNInAs filmcrystalline. The composition of the GaNInAs film can be determined bythe ratio of beam flux of the GaN to InAs. The films can be grown at400° C. in 100 milli-Torr of hydrogen, using an excimer laser at 250milli-joules of power.

Analysis of double crystal X-ray rocking curves of layered structurescan give information on strain in the layer, crystallographicmis-orientations, and crystal defects. In many embodiments, thismeasurement can be non-destructive. FIG. 88 shows a double crystal X-rayrocking curve for a GaNInAs 8802 layer grown on a GaAs 8801 substrate.The layer can be approximately 1500 Å thick. The main peak at 0 degreecan be a peak of the GaAs 8801 substrate. A peak of the GaNInAs 8802layer can be about 2000 arc-seconds from the peak of the GaAs 8801substrate showing that it is near lattice matched to the GaAs. Due tothe thinness of the layer, it can be strained from the mismatch. Thedouble crystal X-ray rocking curve for the GaNInAs films grown on GaAsshow a clear X-ray peak from the GaNInAs indicating the layer is ofcrystalline quality.

Photoluminescence (PL) measurements can be a non-contact,non-destructive method of probing the electronic structure of materials.In many embodiments, an above bandgap energy laser light can be directedonto a sample for photo-excitation of the sample. When photo-excitationis above the bandgap energy of the semiconductor, this can generateelectron-hole pairs and as these pairs return to a lower energy level,the pairs can emit light or luminescence. From PL measurements, one candetermine the energy bandgap, impurity levels and defects, recombinationmechanisms, and the material quality. In particular, room temperature PLmeasurements can be a clear indicator that a material is of highquality. As the temperature of a material is lowered, the luminescenceof the material can get significantly stronger, thus obtaining roomtemperature PL can provide a metric for identifying a good qualitymaterial.

FIG. 89 shows room temperature PL measurements on a GaNInAs 8902 layergrown on a GaAs substrate 8901. The measurement was taken using a 532 nm100 mW laser for photo-excitation and the luminescence was collected bya 1 mm multimode silica optical fiber. The PL spectra show a clear peakof the GaAs substrate 8901 and a broader peak of the GaNInAs 8902 layer.Based on the peak position of the GaNInAs, the bandgap energy is betweenapproximately 0.9 and approximately 1 eV. The clear PL peak from theGaNInAs 8902 layer shows that the layer is a high quality film.

Spectral ellipsometry measurements can be performed on the samples.Spectral ellipsometry is a reflection technique for non-contactinvestigations of the properties of thin metal films, where thereflection polarization states are measured as a function of angle andwavelength. FIG. 90 shows a spectral ellipsometric scan of GaNInAs filmgrown on GaAs. The data determines the real ε₁ and imaginary part ε₂dielectric constant of the material. The imaginary dielectric constantε₂ determines the absorption of the film. The data shows that GaNInAs9001 bandgap energy of the material is about 0.9 eV, corroborating thePL results.

The composition of the GaNInAs films can be determined by energydispersive X-ray spectroscopy (EDS), Rutherford back scattering (RBS),and laser induced breakdown spectroscopy (LIES). EDS is an analysistechnique to determine the composition of films because each element hasan atomic structure with a unique set of X-ray peaks. A high-energy beamof electrons is focused into the sample thus forming electrontransitions between a higher energy shell and a lower energy shell. Theintensity and energy of the X-rays emitted from a specimen can bemeasured by an energy dispersive spectrometer. The X-rays arecharacteristic of energy between the two shells, and this difference canallow a determination of the elemental composition of the sample. TheEDS measurements show that the percentage of nitrogen in the GaNInAsfilms can be approximately 29%. The films were additionally analyzed byRBS measurements to show that they are comprised of high contentnitrogen films. Additionally, the determination of elementalconcentration of GaNInAs semiconductor can be evaluated by laser inducedbreakdown spectroscopy. LIBS offers direct chemical analysis for everyelement in solid materials. Table 35 shows a comparison of the EDS data,the RBS data, and the LIB S data.

TABLE 35 Comparison of EDS vs. RBS vs. LIBS for GaNInAs Films. EDSAtomic RBS Atomic LIBS Atomic Element % % % Average Ga 31.7 24.4 29 28.4N 28.7 22.2 35 28.6 In 22.2 31.1 22 25.1 As 17.4 22.2 15 18.2

To date films with this much nitrogen for near lattice matched GaAs havenot previously been grown.

PLD growth of P-type GaNInAs was grown on N-type GaAs stack. FIG. 91shows current voltage characteristic measured on a curve tracer for theP—GaNInAs/N—GaAs junction, which show that the turn-on voltage of 0.5 Vis low. The excellent diode characteristics show a low turn-on voltageof 0.5 V and also a breakdown voltage greater than 20 V.

There are many possible applications of the GaNInAs materials forelectronic and photonic applications. FIG. 92 shows a generalconfiguration of a cross-sectional view of a bipolar transistorutilizing a GaNInAs base in a vertical stack geometry, which can belattice matched to GaAs. In some embodiments, the structure can be grownepitaxially, ion implanted or fabricated by various means. For avertical bipolar transistor, in many embodiments, a conducting orsemi-insulating GaAs substrate 9201 can be used as the seed crystal tostart the growth of the structure. A highly conducting GaAssub-collector 9202 can be provided (e.g., grown) over GaAs substrate9201, followed by a low doped GaAs collector 9203. A GaNInAs base 9204,which can be of opposite conductivity as the GaAs collector 9203, canthen be provided (e.g., grown) over GaAs collector 9203, followed by aGaAs emitter 9205 which can have the same conductivity as the GaAscollector 9203, and finally a highly conducting GaAs layer 9206.Electrical contact is made to the device via the metalized contact pads:emitter contact 9207, base contact 9208, and collector contact 9209. Thevoltages and currents are applied to the device via the contact pads.Vertical configuration can offer some advantages.

There are other possible device configurations of the GaNInAs materialsfor electronic and photonic applications. The GaNInAs materials with abandgap energy of approximately 1 eV can be useful for solar cells. FIG.93 shows a general configuration for spectrum splitting solar cell whichcan increase energy conversion efficiency by reducing super and subbandgap losses thus utilizing the full spectrum of the sunlight 9300.There are various ways of spectrum splitting, such as using dichroiclenses, mechanical stacking to facilitate the spectrum splitting, or theuse of multi junction solar cells. In such a structure as the multijunction solar cells, a wide bandgap absorbing layer 9301 junction cansit at the top, and absorb most of the short wavelength light 9304,which typically covers ultraviolet and blue radiation light. Theremaining sunlight can pass through the top junction to be absorbed in amid bandgap absorbing layer 9302 junction, and absorb most of the midwavelength light 9305, which typically covers visible light region. Theremaining sunlight then can pass through to be absorbed in a narrowbandgap absorbing layer 9303 junction, and absorb most of the longwavelength light 9306, which typically covers red to infrared lightregions. For an epitaxially grown tandem multi junction solar cells, thedifferent sub cells can require a series connection between the subcells, which can necessitate the use of tunnel junctions to connect thesub cells.

FIG. 94 shows an exemplary sample general configuration for a multijunction solar cell which can be grown on a GaAs (or Ge) substrate. Atriple junction tandem solar cell 9400 structure represents a possibleexemplary embodiment for a high efficiency solar cell. An InGaP topjunction 9401 can be a semiconductor alloy of compositionIn_(0.49)Ga_(0.51)P (InGaP), which can be lattice matched to GaAs. InGaPcan be grown in a disordered phase, ordered phase, or a combination ofthe two. The disordered InGaP phase has a bandgap energy ofapproximately 1.9 eV. The bandgap of the ordered InGaP can beapproximately 1.85 eV. In such a tandem multi junction structure thewide bandgap InGaP top junction 9401 can sit at the top of the structureand absorb most of the short wavelength light, which is light of anenergy greater than approximately 1.9 or 1.85 eV, which typically coversultraviolet and blue radiation light. A GaAs middle junction 9402 canhave a bandgap energy of approximately 1.42 eV, and can absorb light ofan energy greater than approximately 1.42 eV. The final remainingsunlight can be absorbed in a GaNInAs bottom junction 9403, which has abandgap energy of approximately 0.9 to approximately 1.0 eV, and islattice matched to GaAs. The nitrogen content of this layer 9403 can befrom approximately 20% to 35% nitrogen. A final junction 9403 can absorbmost of the long wavelength light with energy greater than approximately1.0 eV. This structure can be grown on a GaAs substrate 9404 seedcrystal. This three junction solar cell 9400 structure can be fullylattice matched.

FIG. 94 shows a second exemplary sample general configuration for amulti junction solar cell which can be grown on a GaAs (or Ge)substrate. The four junction tandem solar cell 9410 structure representsan exemplary embodiment for a high efficiency solar cell. An InGaP topjunction 9401 can be a semiconductor alloy of compositionIn_(0.49)Ga_(0.51)P (InGaP) which can be lattice matched to GaAs. InGaPcan be grown in a disordered phase, ordered phase, or a combination ofthe two. The disordered InGaP phase has a bandgap energy ofapproximately 1.9 eV. The bandgap of the ordered InGaP can beapproximately 1.85 eV. In such a tandem multi junction structure, thewide bandgap InGaP top junction 9401 can sit at the top of the structureand absorbs most of the short wavelength light, i.e., light of energygreater than approximately 1.9 or approximately 1.85 eV, which typicallycovers ultraviolet and blue radiation light. A GaAs middle junction 19412 can have a bandgap energy of approximately 1.42 eV, and can absorbthe remaining sunlight of an energy greater than approximately 1.42 eV.A GaNInAs middle junction 2 9413 can have a bandgap energy of 0.9 to 1.0eV, and is lattice matched to GaAs and the nitrogen content of thislayer can be from 20% to 35% nitrogen and can absorb the remainingsunlight of energy greater than about 1 eV. An additional junction canbe added to the triple junction solar cell 9400, to increase efficiency.A final Ge bottom junction 9414 with an energy bandgap of 0.66 eV canabsorb most of the remaining long wavelength light with energy greaterthan about 0.66 eV. This structure can be grown on a GaAs substrate 9404seed crystal. This four junction solar cell 9410 structure can be fullylattice matched.

To summarize, the devices are fabricated using standard semiconductorprocess techniques. For the PN junction fabrication, a single mask levelfor the etching of the base and ohmic anneals will be used. The processwill use mesa wet-etch and metallization lift off techniques common inHBT fabrication. AuGeNiAu or other metals can be used for the N-typeGaAs materials and Al to P-type GeSn. The junctions of interest are theemitter-base junction and the base-collector junction.

Exemplary Embodiment: Base region with all the above compositionalGe—GeSn grading variations of the base from emitter side to collectorside.

Exemplary Embodiment: Base region including all the variations andinclusion of a GeSn quantum well or GeSn quantum dot structure in thebase region making a light emitting transistor laser.

Exemplary Summary of HBT Parameters: The embodiments described hereincan relate to the following: any bipolar transistor using a Ge base;GeSn base; any bipolar transistor using a compositionally graded Ge—GeSnbase; and/or any light emitting bipolar transistor laser using a GeSnactive region which can include a GeSn quantum well or GeSn quantum dotin the base region.

It should be noted that the values of the bandgap energies can changedue to growth conditions and other factors. The values of the conductionand valence band offsets between dissimilar heterojunctionsemiconductors are used as guidelines and can be different dependent onthe growth conditions, doping levels, and other factors. Theseparameters are also dependent on the temperature of the materials.

Although the embodiments have been described with reference to specificembodiments, it will be understood by those skilled in the art thatvarious changes can be made without departing from the spirit or scopeof the invention. Accordingly, the disclosure of embodiments of theinvention is intended to be illustrative of the scope of the inventionand is not intended to be limiting. It is intended that the scope of theinvention shall be limited only to the extent required by the appendedclaims. For example, to one of ordinary skill in the art, it will bereadily apparent that the methods, processes, and activities describedherein may be comprised of many different activities, procedures and beperformed by many different modules, in many different orders that anyelement of the figures may be modified and that the foregoing discussionof certain of these embodiments does not necessarily represent acomplete description of all possible embodiments.

All elements claimed in any particular claim are essential to theembodiment claimed in that particular claim. Consequently, replacementof one or more claimed elements constitutes reconstruction and notrepair. Additionally, benefits, other advantages, and solutions toproblems have been described with regard to specific embodiments. Thebenefits, advantages, solutions to problems, and any element or elementsthat may cause any benefit, advantage, or solution to occur or becomemore pronounced, however, are not to be construed as critical, required,or essential features or elements of any or all of the claims, unlesssuch benefits, advantages, solutions, or elements are stated in suchclaim.

Moreover, embodiments and limitations disclosed herein are not dedicatedto the public under the doctrine of dedication if the embodiments and/orlimitations: (1) are not expressly claimed in the claims; and (2) are orare potentially equivalents of express elements and/or limitations inthe claims under the doctrine of equivalents.

What is claimed is:
 1. A method of manufacturing a heterojunctionbipolar transistor, the method comprising: providing an emitter/basestack, the emitter/base stack comprising: a substrate; a base over thesubstrate; and an emitter over the base and coupled to the base; forminga collector, the collector comprising at least one of GaN or SiC;performing an epitaxial lift-off process on the emitter/base stack byepitaxially lifting-off the base and the emitter from the substratewhile the emitter remains coupled to the base; and after performing theepitaxial lift-off process, wafer bonding the base to the collectorwhile the emitter remains coupled to the base.
 2. The method of claim 1,wherein: the emitter/base stack further comprises: one or more basecontacts over the base; and one or more emitter contacts over theemitter.
 3. The method of claim 1, wherein: the substrate comprisesGaAs; the emitter/base stack further comprises a separation layerbetween the substrate and the base; and the method further comprisesetching the separation layer located over the substrate beforeperforming the epitaxial lift-off process.
 4. The method of claim 3,wherein: the separation layer comprises AlAs or AlGaAs; and the AlGaAscomprises an Al percentage of greater than or equal to approximately40%.
 5. The method of claim 1, wherein: the base comprises at least oneof: Ge, GeSn, or GaNInAs.
 6. The method of claim 5, wherein: the GeSncomprises a Sn percentage of less than or equal to approximately 20%. 7.The method of claim 1, wherein: the base comprises graded GeSn.
 8. Themethod of claim 1, wherein: the emitter comprises GaAs or InGaP.
 9. Themethod of claim 1, wherein: providing the emitter/base stack comprisesVan der Waals bonding the base to the collector.
 10. The method of claim1, wherein: forming the collector comprises forming a seed layerconfigured to modify at least one of an electrical performance or anadhesion of a base-collector junction between the base and thecollector.
 11. The method of claim 10, wherein: the seed layer comprisesat least one of: Ge, GeSn, GeSiSn, ZnSe, GaAs, or InGaAs.
 12. A methodof manufacturing a heterojunction bipolar transistor, the methodcomprising: providing an emitter/base stack, the emitter/base stackcomprising; a substrate; a separation layer over the substrate; a baseover the separation layer over the substrate; and an emitter over thebase and coupled to the base; etching the separation layer over thesubstrate for removal of the substrate from the emitter/base stack;forming a collector; and wafer bonding the base to the collector. 13.The method of claim 12, wherein: the substrate comprises GaAs; theseparation layer comprises AlAs or AlGaAs; and the AlGaAs comprises anAl percentage of greater than or equal to approximately 40%.
 14. Themethod of claim 12, wherein: the base comprises at least one of one of:Ge, GeSn, or GaNInAs.
 15. The method of claim 14, wherein: the GeSncomprises a Sn percentage of less than or equal to approximately 20%.16. The method of claim 12, wherein: the base comprises graded GeSn. 17.The method of claim 12, wherein: the emitter comprises GaAs or InGaP.18. The method of claim 12, wherein: the collector comprises at leastone of GaN or SiC.
 19. The method of claim 12, wherein: providing theemitter/base stack comprises Van der Waals bonding the base to thecollector.
 20. The method of claim 12, wherein: forming the collectorcomprises forming a seed layer configured to modify at least one of anelectrical performance or an adhesion of a base-collector junctionbetween the base and the collector.
 21. The method of claim 20, wherein:the seed layer comprises at least one of: Ge, GeSn, GeSiSn, ZnSe, GaAs,or InGaAs.